diff --git a/modfile/print.go b/modfile/print.go
new file mode 100644
index 0000000..3bbea38
--- /dev/null
+++ b/modfile/print.go
@@ -0,0 +1,165 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Module file printer.
+
+package modfile
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+)
+
+// Format returns a go.mod file as a byte slice, formatted in standard style.
+func Format(f *FileSyntax) []byte {
+	pr := &printer{}
+	pr.file(f)
+	return pr.Bytes()
+}
+
+// A printer collects the state during printing of a file or expression.
+type printer struct {
+	bytes.Buffer           // output buffer
+	comment      []Comment // pending end-of-line comments
+	margin       int       // left margin (indent), a number of tabs
+}
+
+// printf prints to the buffer.
+func (p *printer) printf(format string, args ...interface{}) {
+	fmt.Fprintf(p, format, args...)
+}
+
+// indent returns the position on the current line, in bytes, 0-indexed.
+func (p *printer) indent() int {
+	b := p.Bytes()
+	n := 0
+	for n < len(b) && b[len(b)-1-n] != '\n' {
+		n++
+	}
+	return n
+}
+
+// newline ends the current line, flushing end-of-line comments.
+func (p *printer) newline() {
+	if len(p.comment) > 0 {
+		p.printf(" ")
+		for i, com := range p.comment {
+			if i > 0 {
+				p.trim()
+				p.printf("\n")
+				for i := 0; i < p.margin; i++ {
+					p.printf("\t")
+				}
+			}
+			p.printf("%s", strings.TrimSpace(com.Token))
+		}
+		p.comment = p.comment[:0]
+	}
+
+	p.trim()
+	p.printf("\n")
+	for i := 0; i < p.margin; i++ {
+		p.printf("\t")
+	}
+}
+
+// trim removes trailing spaces and tabs from the current line.
+func (p *printer) trim() {
+	// Remove trailing spaces and tabs from line we're about to end.
+	b := p.Bytes()
+	n := len(b)
+	for n > 0 && (b[n-1] == '\t' || b[n-1] == ' ') {
+		n--
+	}
+	p.Truncate(n)
+}
+
+// file formats the given file into the print buffer.
+func (p *printer) file(f *FileSyntax) {
+	for _, com := range f.Before {
+		p.printf("%s", strings.TrimSpace(com.Token))
+		p.newline()
+	}
+
+	for i, stmt := range f.Stmt {
+		switch x := stmt.(type) {
+		case *CommentBlock:
+			// comments already handled
+			p.expr(x)
+
+		default:
+			p.expr(x)
+			p.newline()
+		}
+
+		for _, com := range stmt.Comment().After {
+			p.printf("%s", strings.TrimSpace(com.Token))
+			p.newline()
+		}
+
+		if i+1 < len(f.Stmt) {
+			p.newline()
+		}
+	}
+}
+
+func (p *printer) expr(x Expr) {
+	// Emit line-comments preceding this expression.
+	if before := x.Comment().Before; len(before) > 0 {
+		// Want to print a line comment.
+		// Line comments must be at the current margin.
+		p.trim()
+		if p.indent() > 0 {
+			// There's other text on the line. Start a new line.
+			p.printf("\n")
+		}
+		// Re-indent to margin.
+		for i := 0; i < p.margin; i++ {
+			p.printf("\t")
+		}
+		for _, com := range before {
+			p.printf("%s", strings.TrimSpace(com.Token))
+			p.newline()
+		}
+	}
+
+	switch x := x.(type) {
+	default:
+		panic(fmt.Errorf("printer: unexpected type %T", x))
+
+	case *CommentBlock:
+		// done
+
+	case *LParen:
+		p.printf("(")
+	case *RParen:
+		p.printf(")")
+
+	case *Line:
+		sep := ""
+		for _, tok := range x.Token {
+			p.printf("%s%s", sep, tok)
+			sep = " "
+		}
+
+	case *LineBlock:
+		for _, tok := range x.Token {
+			p.printf("%s ", tok)
+		}
+		p.expr(&x.LParen)
+		p.margin++
+		for _, l := range x.Line {
+			p.newline()
+			p.expr(l)
+		}
+		p.margin--
+		p.newline()
+		p.expr(&x.RParen)
+	}
+
+	// Queue end-of-line comments for printing when we
+	// reach the end of the line.
+	p.comment = append(p.comment, x.Comment().Suffix...)
+}
diff --git a/modfile/read.go b/modfile/read.go
new file mode 100644
index 0000000..bfa90a5
--- /dev/null
+++ b/modfile/read.go
@@ -0,0 +1,870 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Module file parser.
+// This is a simplified copy of Google's buildifier parser.
+
+package modfile
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// A Position describes an arbitrary source position in a file, including the
+// file, line, column, and byte offset.
+type Position struct {
+	Line     int // line in input (starting at 1)
+	LineRune int // rune in line (starting at 1)
+	Byte     int // byte in input (starting at 0)
+}
+
+// add returns the position at the end of s, assuming it starts at p.
+func (p Position) add(s string) Position {
+	p.Byte += len(s)
+	if n := strings.Count(s, "\n"); n > 0 {
+		p.Line += n
+		s = s[strings.LastIndex(s, "\n")+1:]
+		p.LineRune = 1
+	}
+	p.LineRune += utf8.RuneCountInString(s)
+	return p
+}
+
+// An Expr represents an input element.
+type Expr interface {
+	// Span returns the start and end position of the expression,
+	// excluding leading or trailing comments.
+	Span() (start, end Position)
+
+	// Comment returns the comments attached to the expression.
+	// This method would normally be named 'Comments' but that
+	// would interfere with embedding a type of the same name.
+	Comment() *Comments
+}
+
+// A Comment represents a single // comment.
+type Comment struct {
+	Start  Position
+	Token  string // without trailing newline
+	Suffix bool   // an end of line (not whole line) comment
+}
+
+// Comments collects the comments associated with an expression.
+type Comments struct {
+	Before []Comment // whole-line comments before this expression
+	Suffix []Comment // end-of-line comments after this expression
+
+	// For top-level expressions only, After lists whole-line
+	// comments following the expression.
+	After []Comment
+}
+
+// Comment returns the receiver. This isn't useful by itself, but
+// a Comments struct is embedded into all the expression
+// implementation types, and this gives each of those a Comment
+// method to satisfy the Expr interface.
+func (c *Comments) Comment() *Comments {
+	return c
+}
+
+// A FileSyntax represents an entire go.mod file.
+type FileSyntax struct {
+	Name string // file path
+	Comments
+	Stmt []Expr
+}
+
+func (x *FileSyntax) Span() (start, end Position) {
+	if len(x.Stmt) == 0 {
+		return
+	}
+	start, _ = x.Stmt[0].Span()
+	_, end = x.Stmt[len(x.Stmt)-1].Span()
+	return start, end
+}
+
+func (x *FileSyntax) addLine(hint Expr, tokens ...string) *Line {
+	if hint == nil {
+		// If no hint given, add to the last statement of the given type.
+	Loop:
+		for i := len(x.Stmt) - 1; i >= 0; i-- {
+			stmt := x.Stmt[i]
+			switch stmt := stmt.(type) {
+			case *Line:
+				if stmt.Token != nil && stmt.Token[0] == tokens[0] {
+					hint = stmt
+					break Loop
+				}
+			case *LineBlock:
+				if stmt.Token[0] == tokens[0] {
+					hint = stmt
+					break Loop
+				}
+			}
+		}
+	}
+
+	if hint != nil {
+		for i, stmt := range x.Stmt {
+			switch stmt := stmt.(type) {
+			case *Line:
+				if stmt == hint {
+					// Convert line to line block.
+					stmt.InBlock = true
+					block := &LineBlock{Token: stmt.Token[:1], Line: []*Line{stmt}}
+					stmt.Token = stmt.Token[1:]
+					x.Stmt[i] = block
+					new := &Line{Token: tokens[1:], InBlock: true}
+					block.Line = append(block.Line, new)
+					return new
+				}
+			case *LineBlock:
+				if stmt == hint {
+					new := &Line{Token: tokens[1:], InBlock: true}
+					stmt.Line = append(stmt.Line, new)
+					return new
+				}
+				for j, line := range stmt.Line {
+					if line == hint {
+						// Add new line after hint.
+						stmt.Line = append(stmt.Line, nil)
+						copy(stmt.Line[j+2:], stmt.Line[j+1:])
+						new := &Line{Token: tokens[1:], InBlock: true}
+						stmt.Line[j+1] = new
+						return new
+					}
+				}
+			}
+		}
+	}
+
+	new := &Line{Token: tokens}
+	x.Stmt = append(x.Stmt, new)
+	return new
+}
+
+func (x *FileSyntax) updateLine(line *Line, tokens ...string) {
+	if line.InBlock {
+		tokens = tokens[1:]
+	}
+	line.Token = tokens
+}
+
+func (x *FileSyntax) removeLine(line *Line) {
+	line.Token = nil
+}
+
+// Cleanup cleans up the file syntax x after any edit operations.
+// To avoid quadratic behavior, removeLine marks the line as dead
+// by setting line.Token = nil but does not remove it from the slice
+// in which it appears. After edits have all been indicated,
+// calling Cleanup cleans out the dead lines.
+func (x *FileSyntax) Cleanup() {
+	w := 0
+	for _, stmt := range x.Stmt {
+		switch stmt := stmt.(type) {
+		case *Line:
+			if stmt.Token == nil {
+				continue
+			}
+		case *LineBlock:
+			ww := 0
+			for _, line := range stmt.Line {
+				if line.Token != nil {
+					stmt.Line[ww] = line
+					ww++
+				}
+			}
+			if ww == 0 {
+				continue
+			}
+			if ww == 1 {
+				// Collapse block into single line.
+				line := &Line{
+					Comments: Comments{
+						Before: commentsAdd(stmt.Before, stmt.Line[0].Before),
+						Suffix: commentsAdd(stmt.Line[0].Suffix, stmt.Suffix),
+						After:  commentsAdd(stmt.Line[0].After, stmt.After),
+					},
+					Token: stringsAdd(stmt.Token, stmt.Line[0].Token),
+				}
+				x.Stmt[w] = line
+				w++
+				continue
+			}
+			stmt.Line = stmt.Line[:ww]
+		}
+		x.Stmt[w] = stmt
+		w++
+	}
+	x.Stmt = x.Stmt[:w]
+}
+
+func commentsAdd(x, y []Comment) []Comment {
+	return append(x[:len(x):len(x)], y...)
+}
+
+func stringsAdd(x, y []string) []string {
+	return append(x[:len(x):len(x)], y...)
+}
+
+// A CommentBlock represents a top-level block of comments separate
+// from any rule.
+type CommentBlock struct {
+	Comments
+	Start Position
+}
+
+func (x *CommentBlock) Span() (start, end Position) {
+	return x.Start, x.Start
+}
+
+// A Line is a single line of tokens.
+type Line struct {
+	Comments
+	Start   Position
+	Token   []string
+	InBlock bool
+	End     Position
+}
+
+func (x *Line) Span() (start, end Position) {
+	return x.Start, x.End
+}
+
+// A LineBlock is a factored block of lines, like
+//
+//	require (
+//		"x"
+//		"y"
+//	)
+//
+type LineBlock struct {
+	Comments
+	Start  Position
+	LParen LParen
+	Token  []string
+	Line   []*Line
+	RParen RParen
+}
+
+func (x *LineBlock) Span() (start, end Position) {
+	return x.Start, x.RParen.Pos.add(")")
+}
+
+// An LParen represents the beginning of a parenthesized line block.
+// It is a place to store suffix comments.
+type LParen struct {
+	Comments
+	Pos Position
+}
+
+func (x *LParen) Span() (start, end Position) {
+	return x.Pos, x.Pos.add(")")
+}
+
+// An RParen represents the end of a parenthesized line block.
+// It is a place to store whole-line (before) comments.
+type RParen struct {
+	Comments
+	Pos Position
+}
+
+func (x *RParen) Span() (start, end Position) {
+	return x.Pos, x.Pos.add(")")
+}
+
+// An input represents a single input file being parsed.
+type input struct {
+	// Lexing state.
+	filename  string    // name of input file, for errors
+	complete  []byte    // entire input
+	remaining []byte    // remaining input
+	token     []byte    // token being scanned
+	lastToken string    // most recently returned token, for error messages
+	pos       Position  // current input position
+	comments  []Comment // accumulated comments
+	endRule   int       // position of end of current rule
+
+	// Parser state.
+	file       *FileSyntax // returned top-level syntax tree
+	parseError error       // error encountered during parsing
+
+	// Comment assignment state.
+	pre  []Expr // all expressions, in preorder traversal
+	post []Expr // all expressions, in postorder traversal
+}
+
+func newInput(filename string, data []byte) *input {
+	return &input{
+		filename:  filename,
+		complete:  data,
+		remaining: data,
+		pos:       Position{Line: 1, LineRune: 1, Byte: 0},
+	}
+}
+
+// parse parses the input file.
+func parse(file string, data []byte) (f *FileSyntax, err error) {
+	in := newInput(file, data)
+	// The parser panics for both routine errors like syntax errors
+	// and for programmer bugs like array index errors.
+	// Turn both into error returns. Catching bug panics is
+	// especially important when processing many files.
+	defer func() {
+		if e := recover(); e != nil {
+			if e == in.parseError {
+				err = in.parseError
+			} else {
+				err = fmt.Errorf("%s:%d:%d: internal error: %v", in.filename, in.pos.Line, in.pos.LineRune, e)
+			}
+		}
+	}()
+
+	// Invoke the parser.
+	in.parseFile()
+	if in.parseError != nil {
+		return nil, in.parseError
+	}
+	in.file.Name = in.filename
+
+	// Assign comments to nearby syntax.
+	in.assignComments()
+
+	return in.file, nil
+}
+
+// Error is called to report an error.
+// The reason s is often "syntax error".
+// Error does not return: it panics.
+func (in *input) Error(s string) {
+	if s == "syntax error" && in.lastToken != "" {
+		s += " near " + in.lastToken
+	}
+	in.parseError = fmt.Errorf("%s:%d:%d: %v", in.filename, in.pos.Line, in.pos.LineRune, s)
+	panic(in.parseError)
+}
+
+// eof reports whether the input has reached end of file.
+func (in *input) eof() bool {
+	return len(in.remaining) == 0
+}
+
+// peekRune returns the next rune in the input without consuming it.
+func (in *input) peekRune() int {
+	if len(in.remaining) == 0 {
+		return 0
+	}
+	r, _ := utf8.DecodeRune(in.remaining)
+	return int(r)
+}
+
+// peekPrefix reports whether the remaining input begins with the given prefix.
+func (in *input) peekPrefix(prefix string) bool {
+	// This is like bytes.HasPrefix(in.remaining, []byte(prefix))
+	// but without the allocation of the []byte copy of prefix.
+	for i := 0; i < len(prefix); i++ {
+		if i >= len(in.remaining) || in.remaining[i] != prefix[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// readRune consumes and returns the next rune in the input.
+func (in *input) readRune() int {
+	if len(in.remaining) == 0 {
+		in.Error("internal lexer error: readRune at EOF")
+	}
+	r, size := utf8.DecodeRune(in.remaining)
+	in.remaining = in.remaining[size:]
+	if r == '\n' {
+		in.pos.Line++
+		in.pos.LineRune = 1
+	} else {
+		in.pos.LineRune++
+	}
+	in.pos.Byte += size
+	return int(r)
+}
+
+type symType struct {
+	pos    Position
+	endPos Position
+	text   string
+}
+
+// startToken marks the beginning of the next input token.
+// It must be followed by a call to endToken, once the token has
+// been consumed using readRune.
+func (in *input) startToken(sym *symType) {
+	in.token = in.remaining
+	sym.text = ""
+	sym.pos = in.pos
+}
+
+// endToken marks the end of an input token.
+// It records the actual token string in sym.text if the caller
+// has not done that already.
+func (in *input) endToken(sym *symType) {
+	if sym.text == "" {
+		tok := string(in.token[:len(in.token)-len(in.remaining)])
+		sym.text = tok
+		in.lastToken = sym.text
+	}
+	sym.endPos = in.pos
+}
+
+// lex is called from the parser to obtain the next input token.
+// It returns the token value (either a rune like '+' or a symbolic token _FOR)
+// and sets val to the data associated with the token.
+// For all our input tokens, the associated data is
+// val.Pos (the position where the token begins)
+// and val.Token (the input string corresponding to the token).
+func (in *input) lex(sym *symType) int {
+	// Skip past spaces, stopping at non-space or EOF.
+	countNL := 0 // number of newlines we've skipped past
+	for !in.eof() {
+		// Skip over spaces. Count newlines so we can give the parser
+		// information about where top-level blank lines are,
+		// for top-level comment assignment.
+		c := in.peekRune()
+		if c == ' ' || c == '\t' || c == '\r' {
+			in.readRune()
+			continue
+		}
+
+		// Comment runs to end of line.
+		if in.peekPrefix("//") {
+			in.startToken(sym)
+
+			// Is this comment the only thing on its line?
+			// Find the last \n before this // and see if it's all
+			// spaces from there to here.
+			i := bytes.LastIndex(in.complete[:in.pos.Byte], []byte("\n"))
+			suffix := len(bytes.TrimSpace(in.complete[i+1:in.pos.Byte])) > 0
+			in.readRune()
+			in.readRune()
+
+			// Consume comment.
+			for len(in.remaining) > 0 && in.readRune() != '\n' {
+			}
+			in.endToken(sym)
+
+			sym.text = strings.TrimRight(sym.text, "\n")
+			in.lastToken = "comment"
+
+			// If we are at top level (not in a statement), hand the comment to
+			// the parser as a _COMMENT token. The grammar is written
+			// to handle top-level comments itself.
+			if !suffix {
+				// Not in a statement. Tell parser about top-level comment.
+				return _COMMENT
+			}
+
+			// Otherwise, save comment for later attachment to syntax tree.
+			if countNL > 1 {
+				in.comments = append(in.comments, Comment{sym.pos, "", false})
+			}
+			in.comments = append(in.comments, Comment{sym.pos, sym.text, suffix})
+			countNL = 1
+			return _EOL
+		}
+
+		if in.peekPrefix("/*") {
+			in.Error(fmt.Sprintf("mod files must use // comments (not /* */ comments)"))
+		}
+
+		// Found non-space non-comment.
+		break
+	}
+
+	// Found the beginning of the next token.
+	in.startToken(sym)
+	defer in.endToken(sym)
+
+	// End of file.
+	if in.eof() {
+		in.lastToken = "EOF"
+		return _EOF
+	}
+
+	// Punctuation tokens.
+	switch c := in.peekRune(); c {
+	case '\n':
+		in.readRune()
+		return c
+
+	case '(':
+		in.readRune()
+		return c
+
+	case ')':
+		in.readRune()
+		return c
+
+	case '"', '`': // quoted string
+		quote := c
+		in.readRune()
+		for {
+			if in.eof() {
+				in.pos = sym.pos
+				in.Error("unexpected EOF in string")
+			}
+			if in.peekRune() == '\n' {
+				in.Error("unexpected newline in string")
+			}
+			c := in.readRune()
+			if c == quote {
+				break
+			}
+			if c == '\\' && quote != '`' {
+				if in.eof() {
+					in.pos = sym.pos
+					in.Error("unexpected EOF in string")
+				}
+				in.readRune()
+			}
+		}
+		in.endToken(sym)
+		return _STRING
+	}
+
+	// Checked all punctuation. Must be identifier token.
+	if c := in.peekRune(); !isIdent(c) {
+		in.Error(fmt.Sprintf("unexpected input character %#q", c))
+	}
+
+	// Scan over identifier.
+	for isIdent(in.peekRune()) {
+		if in.peekPrefix("//") {
+			break
+		}
+		if in.peekPrefix("/*") {
+			in.Error(fmt.Sprintf("mod files must use // comments (not /* */ comments)"))
+		}
+		in.readRune()
+	}
+	return _IDENT
+}
+
+// isIdent reports whether c is an identifier rune.
+// We treat nearly all runes as identifier runes.
+func isIdent(c int) bool {
+	return c != 0 && !unicode.IsSpace(rune(c))
+}
+
+// Comment assignment.
+// We build two lists of all subexpressions, preorder and postorder.
+// The preorder list is ordered by start location, with outer expressions first.
+// The postorder list is ordered by end location, with outer expressions last.
+// We use the preorder list to assign each whole-line comment to the syntax
+// immediately following it, and we use the postorder list to assign each
+// end-of-line comment to the syntax immediately preceding it.
+
+// order walks the expression adding it and its subexpressions to the
+// preorder and postorder lists.
+func (in *input) order(x Expr) {
+	if x != nil {
+		in.pre = append(in.pre, x)
+	}
+	switch x := x.(type) {
+	default:
+		panic(fmt.Errorf("order: unexpected type %T", x))
+	case nil:
+		// nothing
+	case *LParen, *RParen:
+		// nothing
+	case *CommentBlock:
+		// nothing
+	case *Line:
+		// nothing
+	case *FileSyntax:
+		for _, stmt := range x.Stmt {
+			in.order(stmt)
+		}
+	case *LineBlock:
+		in.order(&x.LParen)
+		for _, l := range x.Line {
+			in.order(l)
+		}
+		in.order(&x.RParen)
+	}
+	if x != nil {
+		in.post = append(in.post, x)
+	}
+}
+
+// assignComments attaches comments to nearby syntax.
+func (in *input) assignComments() {
+	const debug = false
+
+	// Generate preorder and postorder lists.
+	in.order(in.file)
+
+	// Split into whole-line comments and suffix comments.
+	var line, suffix []Comment
+	for _, com := range in.comments {
+		if com.Suffix {
+			suffix = append(suffix, com)
+		} else {
+			line = append(line, com)
+		}
+	}
+
+	if debug {
+		for _, c := range line {
+			fmt.Fprintf(os.Stderr, "LINE %q :%d:%d #%d\n", c.Token, c.Start.Line, c.Start.LineRune, c.Start.Byte)
+		}
+	}
+
+	// Assign line comments to syntax immediately following.
+	for _, x := range in.pre {
+		start, _ := x.Span()
+		if debug {
+			fmt.Printf("pre %T :%d:%d #%d\n", x, start.Line, start.LineRune, start.Byte)
+		}
+		xcom := x.Comment()
+		for len(line) > 0 && start.Byte >= line[0].Start.Byte {
+			if debug {
+				fmt.Fprintf(os.Stderr, "ASSIGN LINE %q #%d\n", line[0].Token, line[0].Start.Byte)
+			}
+			xcom.Before = append(xcom.Before, line[0])
+			line = line[1:]
+		}
+	}
+
+	// Remaining line comments go at end of file.
+	in.file.After = append(in.file.After, line...)
+
+	if debug {
+		for _, c := range suffix {
+			fmt.Fprintf(os.Stderr, "SUFFIX %q :%d:%d #%d\n", c.Token, c.Start.Line, c.Start.LineRune, c.Start.Byte)
+		}
+	}
+
+	// Assign suffix comments to syntax immediately before.
+	for i := len(in.post) - 1; i >= 0; i-- {
+		x := in.post[i]
+
+		start, end := x.Span()
+		if debug {
+			fmt.Printf("post %T :%d:%d #%d :%d:%d #%d\n", x, start.Line, start.LineRune, start.Byte, end.Line, end.LineRune, end.Byte)
+		}
+
+		// Do not assign suffix comments to end of line block or whole file.
+		// Instead assign them to the last element inside.
+		switch x.(type) {
+		case *FileSyntax:
+			continue
+		}
+
+		// Do not assign suffix comments to something that starts
+		// on an earlier line, so that in
+		//
+		//	x ( y
+		//		z ) // comment
+		//
+		// we assign the comment to z and not to x ( ... ).
+		if start.Line != end.Line {
+			continue
+		}
+		xcom := x.Comment()
+		for len(suffix) > 0 && end.Byte <= suffix[len(suffix)-1].Start.Byte {
+			if debug {
+				fmt.Fprintf(os.Stderr, "ASSIGN SUFFIX %q #%d\n", suffix[len(suffix)-1].Token, suffix[len(suffix)-1].Start.Byte)
+			}
+			xcom.Suffix = append(xcom.Suffix, suffix[len(suffix)-1])
+			suffix = suffix[:len(suffix)-1]
+		}
+	}
+
+	// We assigned suffix comments in reverse.
+	// If multiple suffix comments were appended to the same
+	// expression node, they are now in reverse. Fix that.
+	for _, x := range in.post {
+		reverseComments(x.Comment().Suffix)
+	}
+
+	// Remaining suffix comments go at beginning of file.
+	in.file.Before = append(in.file.Before, suffix...)
+}
+
+// reverseComments reverses the []Comment list.
+func reverseComments(list []Comment) {
+	for i, j := 0, len(list)-1; i < j; i, j = i+1, j-1 {
+		list[i], list[j] = list[j], list[i]
+	}
+}
+
+func (in *input) parseFile() {
+	in.file = new(FileSyntax)
+	var sym symType
+	var cb *CommentBlock
+	for {
+		tok := in.lex(&sym)
+		switch tok {
+		case '\n':
+			if cb != nil {
+				in.file.Stmt = append(in.file.Stmt, cb)
+				cb = nil
+			}
+		case _COMMENT:
+			if cb == nil {
+				cb = &CommentBlock{Start: sym.pos}
+			}
+			com := cb.Comment()
+			com.Before = append(com.Before, Comment{Start: sym.pos, Token: sym.text})
+		case _EOF:
+			if cb != nil {
+				in.file.Stmt = append(in.file.Stmt, cb)
+			}
+			return
+		default:
+			in.parseStmt(&sym)
+			if cb != nil {
+				in.file.Stmt[len(in.file.Stmt)-1].Comment().Before = cb.Before
+				cb = nil
+			}
+		}
+	}
+}
+
+func (in *input) parseStmt(sym *symType) {
+	start := sym.pos
+	end := sym.endPos
+	token := []string{sym.text}
+	for {
+		tok := in.lex(sym)
+		switch tok {
+		case '\n', _EOF, _EOL:
+			in.file.Stmt = append(in.file.Stmt, &Line{
+				Start: start,
+				Token: token,
+				End:   end,
+			})
+			return
+		case '(':
+			in.file.Stmt = append(in.file.Stmt, in.parseLineBlock(start, token, sym))
+			return
+		default:
+			token = append(token, sym.text)
+			end = sym.endPos
+		}
+	}
+}
+
+func (in *input) parseLineBlock(start Position, token []string, sym *symType) *LineBlock {
+	x := &LineBlock{
+		Start:  start,
+		Token:  token,
+		LParen: LParen{Pos: sym.pos},
+	}
+	var comments []Comment
+	for {
+		tok := in.lex(sym)
+		switch tok {
+		case _EOL:
+			// ignore
+		case '\n':
+			if len(comments) == 0 && len(x.Line) > 0 || len(comments) > 0 && comments[len(comments)-1].Token != "" {
+				comments = append(comments, Comment{})
+			}
+		case _COMMENT:
+			comments = append(comments, Comment{Start: sym.pos, Token: sym.text})
+		case _EOF:
+			in.Error(fmt.Sprintf("syntax error (unterminated block started at %s:%d:%d)", in.filename, x.Start.Line, x.Start.LineRune))
+		case ')':
+			x.RParen.Before = comments
+			x.RParen.Pos = sym.pos
+			tok = in.lex(sym)
+			if tok != '\n' && tok != _EOF && tok != _EOL {
+				in.Error("syntax error (expected newline after closing paren)")
+			}
+			return x
+		default:
+			l := in.parseLine(sym)
+			x.Line = append(x.Line, l)
+			l.Comment().Before = comments
+			comments = nil
+		}
+	}
+}
+
+func (in *input) parseLine(sym *symType) *Line {
+	start := sym.pos
+	end := sym.endPos
+	token := []string{sym.text}
+	for {
+		tok := in.lex(sym)
+		switch tok {
+		case '\n', _EOF, _EOL:
+			return &Line{
+				Start:   start,
+				Token:   token,
+				End:     end,
+				InBlock: true,
+			}
+		default:
+			token = append(token, sym.text)
+			end = sym.endPos
+		}
+	}
+}
+
+const (
+	_EOF = -(1 + iota)
+	_EOL
+	_IDENT
+	_STRING
+	_COMMENT
+)
+
+var (
+	slashSlash = []byte("//")
+	moduleStr  = []byte("module")
+)
+
+// ModulePath returns the module path from the gomod file text.
+// If it cannot find a module path, it returns an empty string.
+// It is tolerant of unrelated problems in the go.mod file.
+func ModulePath(mod []byte) string {
+	for len(mod) > 0 {
+		line := mod
+		mod = nil
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, mod = line[:i], line[i+1:]
+		}
+		if i := bytes.Index(line, slashSlash); i >= 0 {
+			line = line[:i]
+		}
+		line = bytes.TrimSpace(line)
+		if !bytes.HasPrefix(line, moduleStr) {
+			continue
+		}
+		line = line[len(moduleStr):]
+		n := len(line)
+		line = bytes.TrimSpace(line)
+		if len(line) == n || len(line) == 0 {
+			continue
+		}
+
+		if line[0] == '"' || line[0] == '`' {
+			p, err := strconv.Unquote(string(line))
+			if err != nil {
+				return "" // malformed quoted string or multiline module path
+			}
+			return p
+		}
+
+		return string(line)
+	}
+	return "" // missing module path
+}
diff --git a/modfile/read_test.go b/modfile/read_test.go
new file mode 100644
index 0000000..3240130
--- /dev/null
+++ b/modfile/read_test.go
@@ -0,0 +1,388 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfile
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// exists reports whether the named file exists.
+func exists(name string) bool {
+	_, err := os.Stat(name)
+	return err == nil
+}
+
+// Test that reading and then writing the golden files
+// does not change their output.
+func TestPrintGolden(t *testing.T) {
+	outs, err := filepath.Glob("testdata/*.golden")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, out := range outs {
+		testPrint(t, out, out)
+	}
+}
+
+// testPrint is a helper for testing the printer.
+// It reads the file named in, reformats it, and compares
+// the result to the file named out.
+func testPrint(t *testing.T, in, out string) {
+	data, err := ioutil.ReadFile(in)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	golden, err := ioutil.ReadFile(out)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	base := "testdata/" + filepath.Base(in)
+	f, err := parse(in, data)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	ndata := Format(f)
+
+	if !bytes.Equal(ndata, golden) {
+		t.Errorf("formatted %s incorrectly: diff shows -golden, +ours", base)
+		tdiff(t, string(golden), string(ndata))
+		return
+	}
+}
+
+func TestParseLax(t *testing.T) {
+	badFile := []byte(`module m
+		surprise attack
+		x y (
+			z
+		)
+		exclude v1.2.3
+		replace <-!!!
+	`)
+	_, err := ParseLax("file", badFile, nil)
+	if err != nil {
+		t.Fatalf("ParseLax did not ignore irrelevant errors: %v", err)
+	}
+}
+
+// Test that when files in the testdata directory are parsed
+// and printed and parsed again, we get the same parse tree
+// both times.
+func TestPrintParse(t *testing.T) {
+	outs, err := filepath.Glob("testdata/*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, out := range outs {
+		data, err := ioutil.ReadFile(out)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		base := "testdata/" + filepath.Base(out)
+		f, err := parse(base, data)
+		if err != nil {
+			t.Errorf("parsing original: %v", err)
+			continue
+		}
+
+		ndata := Format(f)
+		f2, err := parse(base, ndata)
+		if err != nil {
+			t.Errorf("parsing reformatted: %v", err)
+			continue
+		}
+
+		eq := eqchecker{file: base}
+		if err := eq.check(f, f2); err != nil {
+			t.Errorf("not equal (parse/Format/parse): %v", err)
+		}
+
+		pf1, err := Parse(base, data, nil)
+		if err != nil {
+			switch base {
+			case "testdata/replace2.in", "testdata/gopkg.in.golden":
+				t.Errorf("should parse %v: %v", base, err)
+			}
+		}
+		if err == nil {
+			pf2, err := Parse(base, ndata, nil)
+			if err != nil {
+				t.Errorf("Parsing reformatted: %v", err)
+				continue
+			}
+			eq := eqchecker{file: base}
+			if err := eq.check(pf1, pf2); err != nil {
+				t.Errorf("not equal (parse/Format/Parse): %v", err)
+			}
+
+			ndata2, err := pf1.Format()
+			if err != nil {
+				t.Errorf("reformat: %v", err)
+			}
+			pf3, err := Parse(base, ndata2, nil)
+			if err != nil {
+				t.Errorf("Parsing reformatted2: %v", err)
+				continue
+			}
+			eq = eqchecker{file: base}
+			if err := eq.check(pf1, pf3); err != nil {
+				t.Errorf("not equal (Parse/Format/Parse): %v", err)
+			}
+			ndata = ndata2
+		}
+
+		if strings.HasSuffix(out, ".in") {
+			golden, err := ioutil.ReadFile(strings.TrimSuffix(out, ".in") + ".golden")
+			if err != nil {
+				t.Error(err)
+				continue
+			}
+			if !bytes.Equal(ndata, golden) {
+				t.Errorf("formatted %s incorrectly: diff shows -golden, +ours", base)
+				tdiff(t, string(golden), string(ndata))
+				return
+			}
+		}
+	}
+}
+
+// An eqchecker holds state for checking the equality of two parse trees.
+type eqchecker struct {
+	file string
+	pos  Position
+}
+
+// errorf returns an error described by the printf-style format and arguments,
+// inserting the current file position before the error text.
+func (eq *eqchecker) errorf(format string, args ...interface{}) error {
+	return fmt.Errorf("%s:%d: %s", eq.file, eq.pos.Line,
+		fmt.Sprintf(format, args...))
+}
+
+// check checks that v and w represent the same parse tree.
+// If not, it returns an error describing the first difference.
+func (eq *eqchecker) check(v, w interface{}) error {
+	return eq.checkValue(reflect.ValueOf(v), reflect.ValueOf(w))
+}
+
+var (
+	posType      = reflect.TypeOf(Position{})
+	commentsType = reflect.TypeOf(Comments{})
+)
+
+// checkValue checks that v and w represent the same parse tree.
+// If not, it returns an error describing the first difference.
+func (eq *eqchecker) checkValue(v, w reflect.Value) error {
+	// inner returns the innermost expression for v.
+	// if v is a non-nil interface value, it returns the concrete
+	// value in the interface.
+	inner := func(v reflect.Value) reflect.Value {
+		for {
+			if v.Kind() == reflect.Interface && !v.IsNil() {
+				v = v.Elem()
+				continue
+			}
+			break
+		}
+		return v
+	}
+
+	v = inner(v)
+	w = inner(w)
+	if v.Kind() == reflect.Invalid && w.Kind() == reflect.Invalid {
+		return nil
+	}
+	if v.Kind() == reflect.Invalid {
+		return eq.errorf("nil interface became %s", w.Type())
+	}
+	if w.Kind() == reflect.Invalid {
+		return eq.errorf("%s became nil interface", v.Type())
+	}
+
+	if v.Type() != w.Type() {
+		return eq.errorf("%s became %s", v.Type(), w.Type())
+	}
+
+	if p, ok := v.Interface().(Expr); ok {
+		eq.pos, _ = p.Span()
+	}
+
+	switch v.Kind() {
+	default:
+		return eq.errorf("unexpected type %s", v.Type())
+
+	case reflect.Bool, reflect.Int, reflect.String:
+		vi := v.Interface()
+		wi := w.Interface()
+		if vi != wi {
+			return eq.errorf("%v became %v", vi, wi)
+		}
+
+	case reflect.Slice:
+		vl := v.Len()
+		wl := w.Len()
+		for i := 0; i < vl || i < wl; i++ {
+			if i >= vl {
+				return eq.errorf("unexpected %s", w.Index(i).Type())
+			}
+			if i >= wl {
+				return eq.errorf("missing %s", v.Index(i).Type())
+			}
+			if err := eq.checkValue(v.Index(i), w.Index(i)); err != nil {
+				return err
+			}
+		}
+
+	case reflect.Struct:
+		// Fields in struct must match.
+		t := v.Type()
+		n := t.NumField()
+		for i := 0; i < n; i++ {
+			tf := t.Field(i)
+			switch {
+			default:
+				if err := eq.checkValue(v.Field(i), w.Field(i)); err != nil {
+					return err
+				}
+
+			case tf.Type == posType: // ignore positions
+			case tf.Type == commentsType: // ignore comment assignment
+			}
+		}
+
+	case reflect.Ptr, reflect.Interface:
+		if v.IsNil() != w.IsNil() {
+			if v.IsNil() {
+				return eq.errorf("unexpected %s", w.Elem().Type())
+			}
+			return eq.errorf("missing %s", v.Elem().Type())
+		}
+		if err := eq.checkValue(v.Elem(), w.Elem()); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// diff returns the output of running diff on b1 and b2.
+func diff(b1, b2 []byte) (data []byte, err error) {
+	f1, err := ioutil.TempFile("", "testdiff")
+	if err != nil {
+		return nil, err
+	}
+	defer os.Remove(f1.Name())
+	defer f1.Close()
+
+	f2, err := ioutil.TempFile("", "testdiff")
+	if err != nil {
+		return nil, err
+	}
+	defer os.Remove(f2.Name())
+	defer f2.Close()
+
+	f1.Write(b1)
+	f2.Write(b2)
+
+	data, err = exec.Command("diff", "-u", f1.Name(), f2.Name()).CombinedOutput()
+	if len(data) > 0 {
+		// diff exits with a non-zero status when the files don't match.
+		// Ignore that failure as long as we get output.
+		err = nil
+	}
+	return
+}
+
+// tdiff logs the diff output to t.Error.
+func tdiff(t *testing.T, a, b string) {
+	data, err := diff([]byte(a), []byte(b))
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	t.Error(string(data))
+}
+
+var modulePathTests = []struct {
+	input    []byte
+	expected string
+}{
+	{input: []byte("module \"github.com/rsc/vgotest\""), expected: "github.com/rsc/vgotest"},
+	{input: []byte("module github.com/rsc/vgotest"), expected: "github.com/rsc/vgotest"},
+	{input: []byte("module  \"github.com/rsc/vgotest\""), expected: "github.com/rsc/vgotest"},
+	{input: []byte("module  github.com/rsc/vgotest"), expected: "github.com/rsc/vgotest"},
+	{input: []byte("module `github.com/rsc/vgotest`"), expected: "github.com/rsc/vgotest"},
+	{input: []byte("module \"github.com/rsc/vgotest/v2\""), expected: "github.com/rsc/vgotest/v2"},
+	{input: []byte("module github.com/rsc/vgotest/v2"), expected: "github.com/rsc/vgotest/v2"},
+	{input: []byte("module \"gopkg.in/yaml.v2\""), expected: "gopkg.in/yaml.v2"},
+	{input: []byte("module gopkg.in/yaml.v2"), expected: "gopkg.in/yaml.v2"},
+	{input: []byte("module \"gopkg.in/check.v1\"\n"), expected: "gopkg.in/check.v1"},
+	{input: []byte("module \"gopkg.in/check.v1\n\""), expected: ""},
+	{input: []byte("module gopkg.in/check.v1\n"), expected: "gopkg.in/check.v1"},
+	{input: []byte("module \"gopkg.in/check.v1\"\r\n"), expected: "gopkg.in/check.v1"},
+	{input: []byte("module gopkg.in/check.v1\r\n"), expected: "gopkg.in/check.v1"},
+	{input: []byte("module \"gopkg.in/check.v1\"\n\n"), expected: "gopkg.in/check.v1"},
+	{input: []byte("module gopkg.in/check.v1\n\n"), expected: "gopkg.in/check.v1"},
+	{input: []byte("module \n\"gopkg.in/check.v1\"\n\n"), expected: ""},
+	{input: []byte("module \ngopkg.in/check.v1\n\n"), expected: ""},
+	{input: []byte("module \"gopkg.in/check.v1\"asd"), expected: ""},
+	{input: []byte("module \n\"gopkg.in/check.v1\"\n\n"), expected: ""},
+	{input: []byte("module \ngopkg.in/check.v1\n\n"), expected: ""},
+	{input: []byte("module \"gopkg.in/check.v1\"asd"), expected: ""},
+	{input: []byte("module  \nmodule a/b/c "), expected: "a/b/c"},
+	{input: []byte("module \"   \""), expected: "   "},
+	{input: []byte("module   "), expected: ""},
+	{input: []byte("module \"  a/b/c  \""), expected: "  a/b/c  "},
+	{input: []byte("module \"github.com/rsc/vgotest1\" // with a comment"), expected: "github.com/rsc/vgotest1"},
+}
+
+func TestModulePath(t *testing.T) {
+	for _, test := range modulePathTests {
+		t.Run(string(test.input), func(t *testing.T) {
+			result := ModulePath(test.input)
+			if result != test.expected {
+				t.Fatalf("ModulePath(%q): %s, want %s", string(test.input), result, test.expected)
+			}
+		})
+	}
+}
+
+func TestGoVersion(t *testing.T) {
+	for _, test := range []struct {
+		desc, input string
+		ok          bool
+	}{
+		{desc: "empty", input: "module m\ngo \n", ok: false},
+		{desc: "one", input: "module m\ngo 1\n", ok: false},
+		{desc: "two", input: "module m\ngo 1.22\n", ok: true},
+		{desc: "three", input: "module m\ngo 1.22.333", ok: false},
+		{desc: "before", input: "module m\ngo v1.2\n", ok: false},
+		{desc: "after", input: "module m\ngo 1.2rc1\n", ok: false},
+		{desc: "space", input: "module m\ngo 1.2 3.4\n", ok: false},
+	} {
+		t.Run(test.desc, func(t *testing.T) {
+			if _, err := Parse("go.mod", []byte(test.input), nil); err == nil && !test.ok {
+				t.Error("unexpected success")
+			} else if err != nil && test.ok {
+				t.Errorf("unexpected error: %v", err)
+			}
+		})
+	}
+}
diff --git a/modfile/rule.go b/modfile/rule.go
new file mode 100644
index 0000000..95fefec
--- /dev/null
+++ b/modfile/rule.go
@@ -0,0 +1,766 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfile
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+
+	"golang.org/x/mod/internal/lazyregexp"
+	"golang.org/x/mod/module"
+)
+
+// A File is the parsed, interpreted form of a go.mod file.
+type File struct {
+	Module  *Module
+	Go      *Go
+	Require []*Require
+	Exclude []*Exclude
+	Replace []*Replace
+
+	Syntax *FileSyntax
+}
+
+// A Module is the module statement.
+type Module struct {
+	Mod    module.Version
+	Syntax *Line
+}
+
+// A Go is the go statement.
+type Go struct {
+	Version string // "1.23"
+	Syntax  *Line
+}
+
+// A Require is a single require statement.
+type Require struct {
+	Mod      module.Version
+	Indirect bool // has "// indirect" comment
+	Syntax   *Line
+}
+
+// An Exclude is a single exclude statement.
+type Exclude struct {
+	Mod    module.Version
+	Syntax *Line
+}
+
+// A Replace is a single replace statement.
+type Replace struct {
+	Old    module.Version
+	New    module.Version
+	Syntax *Line
+}
+
+func (f *File) AddModuleStmt(path string) error {
+	if f.Syntax == nil {
+		f.Syntax = new(FileSyntax)
+	}
+	if f.Module == nil {
+		f.Module = &Module{
+			Mod:    module.Version{Path: path},
+			Syntax: f.Syntax.addLine(nil, "module", AutoQuote(path)),
+		}
+	} else {
+		f.Module.Mod.Path = path
+		f.Syntax.updateLine(f.Module.Syntax, "module", AutoQuote(path))
+	}
+	return nil
+}
+
+func (f *File) AddComment(text string) {
+	if f.Syntax == nil {
+		f.Syntax = new(FileSyntax)
+	}
+	f.Syntax.Stmt = append(f.Syntax.Stmt, &CommentBlock{
+		Comments: Comments{
+			Before: []Comment{
+				{
+					Token: text,
+				},
+			},
+		},
+	})
+}
+
+type VersionFixer func(path, version string) (string, error)
+
+// Parse parses the data, reported in errors as being from file,
+// into a File struct. It applies fix, if non-nil, to canonicalize all module versions found.
+func Parse(file string, data []byte, fix VersionFixer) (*File, error) {
+	return parseToFile(file, data, fix, true)
+}
+
+// ParseLax is like Parse but ignores unknown statements.
+// It is used when parsing go.mod files other than the main module,
+// under the theory that most statement types we add in the future will
+// only apply in the main module, like exclude and replace,
+// and so we get better gradual deployments if old go commands
+// simply ignore those statements when found in go.mod files
+// in dependencies.
+func ParseLax(file string, data []byte, fix VersionFixer) (*File, error) {
+	return parseToFile(file, data, fix, false)
+}
+
+func parseToFile(file string, data []byte, fix VersionFixer, strict bool) (*File, error) {
+	fs, err := parse(file, data)
+	if err != nil {
+		return nil, err
+	}
+	f := &File{
+		Syntax: fs,
+	}
+
+	var errs bytes.Buffer
+	for _, x := range fs.Stmt {
+		switch x := x.(type) {
+		case *Line:
+			f.add(&errs, x, x.Token[0], x.Token[1:], fix, strict)
+
+		case *LineBlock:
+			if len(x.Token) > 1 {
+				if strict {
+					fmt.Fprintf(&errs, "%s:%d: unknown block type: %s\n", file, x.Start.Line, strings.Join(x.Token, " "))
+				}
+				continue
+			}
+			switch x.Token[0] {
+			default:
+				if strict {
+					fmt.Fprintf(&errs, "%s:%d: unknown block type: %s\n", file, x.Start.Line, strings.Join(x.Token, " "))
+				}
+				continue
+			case "module", "require", "exclude", "replace":
+				for _, l := range x.Line {
+					f.add(&errs, l, x.Token[0], l.Token, fix, strict)
+				}
+			}
+		}
+	}
+
+	if errs.Len() > 0 {
+		return nil, errors.New(strings.TrimRight(errs.String(), "\n"))
+	}
+	return f, nil
+}
+
+var GoVersionRE = lazyregexp.New(`^([1-9][0-9]*)\.(0|[1-9][0-9]*)$`)
+
+func (f *File) add(errs *bytes.Buffer, line *Line, verb string, args []string, fix VersionFixer, strict bool) {
+	// If strict is false, this module is a dependency.
+	// We ignore all unknown directives as well as main-module-only
+	// directives like replace and exclude. It will work better for
+	// forward compatibility if we can depend on modules that have unknown
+	// statements (presumed relevant only when acting as the main module)
+	// and simply ignore those statements.
+	if !strict {
+		switch verb {
+		case "module", "require", "go":
+			// want these even for dependency go.mods
+		default:
+			return
+		}
+	}
+
+	switch verb {
+	default:
+		fmt.Fprintf(errs, "%s:%d: unknown directive: %s\n", f.Syntax.Name, line.Start.Line, verb)
+
+	case "go":
+		if f.Go != nil {
+			fmt.Fprintf(errs, "%s:%d: repeated go statement\n", f.Syntax.Name, line.Start.Line)
+			return
+		}
+		if len(args) != 1 || !GoVersionRE.MatchString(args[0]) {
+			fmt.Fprintf(errs, "%s:%d: usage: go 1.23\n", f.Syntax.Name, line.Start.Line)
+			return
+		}
+		f.Go = &Go{Syntax: line}
+		f.Go.Version = args[0]
+	case "module":
+		if f.Module != nil {
+			fmt.Fprintf(errs, "%s:%d: repeated module statement\n", f.Syntax.Name, line.Start.Line)
+			return
+		}
+		f.Module = &Module{Syntax: line}
+		if len(args) != 1 {
+
+			fmt.Fprintf(errs, "%s:%d: usage: module module/path\n", f.Syntax.Name, line.Start.Line)
+			return
+		}
+		s, err := parseString(&args[0])
+		if err != nil {
+			fmt.Fprintf(errs, "%s:%d: invalid quoted string: %v\n", f.Syntax.Name, line.Start.Line, err)
+			return
+		}
+		f.Module.Mod = module.Version{Path: s}
+	case "require", "exclude":
+		if len(args) != 2 {
+			fmt.Fprintf(errs, "%s:%d: usage: %s module/path v1.2.3\n", f.Syntax.Name, line.Start.Line, verb)
+			return
+		}
+		s, err := parseString(&args[0])
+		if err != nil {
+			fmt.Fprintf(errs, "%s:%d: invalid quoted string: %v\n", f.Syntax.Name, line.Start.Line, err)
+			return
+		}
+		v, err := parseVersion(verb, s, &args[1], fix)
+		if err != nil {
+			fmt.Fprintf(errs, "%s:%d: %v\n", f.Syntax.Name, line.Start.Line, err)
+			return
+		}
+		pathMajor, err := modulePathMajor(s)
+		if err != nil {
+			fmt.Fprintf(errs, "%s:%d: %v\n", f.Syntax.Name, line.Start.Line, err)
+			return
+		}
+		if err := module.CheckPathMajor(v, pathMajor); err != nil {
+			fmt.Fprintf(errs, "%s:%d: %v\n", f.Syntax.Name, line.Start.Line, &Error{Verb: verb, ModPath: s, Err: err})
+			return
+		}
+		if verb == "require" {
+			f.Require = append(f.Require, &Require{
+				Mod:      module.Version{Path: s, Version: v},
+				Syntax:   line,
+				Indirect: isIndirect(line),
+			})
+		} else {
+			f.Exclude = append(f.Exclude, &Exclude{
+				Mod:    module.Version{Path: s, Version: v},
+				Syntax: line,
+			})
+		}
+	case "replace":
+		arrow := 2
+		if len(args) >= 2 && args[1] == "=>" {
+			arrow = 1
+		}
+		if len(args) < arrow+2 || len(args) > arrow+3 || args[arrow] != "=>" {
+			fmt.Fprintf(errs, "%s:%d: usage: %s module/path [v1.2.3] => other/module v1.4\n\t or %s module/path [v1.2.3] => ../local/directory\n", f.Syntax.Name, line.Start.Line, verb, verb)
+			return
+		}
+		s, err := parseString(&args[0])
+		if err != nil {
+			fmt.Fprintf(errs, "%s:%d: invalid quoted string: %v\n", f.Syntax.Name, line.Start.Line, err)
+			return
+		}
+		pathMajor, err := modulePathMajor(s)
+		if err != nil {
+			fmt.Fprintf(errs, "%s:%d: %v\n", f.Syntax.Name, line.Start.Line, err)
+			return
+		}
+		var v string
+		if arrow == 2 {
+			v, err = parseVersion(verb, s, &args[1], fix)
+			if err != nil {
+				fmt.Fprintf(errs, "%s:%d: %v\n", f.Syntax.Name, line.Start.Line, err)
+				return
+			}
+			if err := module.CheckPathMajor(v, pathMajor); err != nil {
+				fmt.Fprintf(errs, "%s:%d: %v\n", f.Syntax.Name, line.Start.Line, &Error{Verb: verb, ModPath: s, Err: err})
+				return
+			}
+		}
+		ns, err := parseString(&args[arrow+1])
+		if err != nil {
+			fmt.Fprintf(errs, "%s:%d: invalid quoted string: %v\n", f.Syntax.Name, line.Start.Line, err)
+			return
+		}
+		nv := ""
+		if len(args) == arrow+2 {
+			if !IsDirectoryPath(ns) {
+				fmt.Fprintf(errs, "%s:%d: replacement module without version must be directory path (rooted or starting with ./ or ../)\n", f.Syntax.Name, line.Start.Line)
+				return
+			}
+			if filepath.Separator == '/' && strings.Contains(ns, `\`) {
+				fmt.Fprintf(errs, "%s:%d: replacement directory appears to be Windows path (on a non-windows system)\n", f.Syntax.Name, line.Start.Line)
+				return
+			}
+		}
+		if len(args) == arrow+3 {
+			nv, err = parseVersion(verb, ns, &args[arrow+2], fix)
+			if err != nil {
+				fmt.Fprintf(errs, "%s:%d: %v\n", f.Syntax.Name, line.Start.Line, err)
+				return
+			}
+			if IsDirectoryPath(ns) {
+				fmt.Fprintf(errs, "%s:%d: replacement module directory path %q cannot have version\n", f.Syntax.Name, line.Start.Line, ns)
+				return
+			}
+		}
+		f.Replace = append(f.Replace, &Replace{
+			Old:    module.Version{Path: s, Version: v},
+			New:    module.Version{Path: ns, Version: nv},
+			Syntax: line,
+		})
+	}
+}
+
+// isIndirect reports whether line has a "// indirect" comment,
+// meaning it is in go.mod only for its effect on indirect dependencies,
+// so that it can be dropped entirely once the effective version of the
+// indirect dependency reaches the given minimum version.
+func isIndirect(line *Line) bool {
+	if len(line.Suffix) == 0 {
+		return false
+	}
+	f := strings.Fields(line.Suffix[0].Token)
+	return (len(f) == 2 && f[1] == "indirect" || len(f) > 2 && f[1] == "indirect;") && f[0] == "//"
+}
+
+// setIndirect sets line to have (or not have) a "// indirect" comment.
+func setIndirect(line *Line, indirect bool) {
+	if isIndirect(line) == indirect {
+		return
+	}
+	if indirect {
+		// Adding comment.
+		if len(line.Suffix) == 0 {
+			// New comment.
+			line.Suffix = []Comment{{Token: "// indirect", Suffix: true}}
+			return
+		}
+		// Insert at beginning of existing comment.
+		com := &line.Suffix[0]
+		space := " "
+		if len(com.Token) > 2 && com.Token[2] == ' ' || com.Token[2] == '\t' {
+			space = ""
+		}
+		com.Token = "// indirect;" + space + com.Token[2:]
+		return
+	}
+
+	// Removing comment.
+	f := strings.Fields(line.Suffix[0].Token)
+	if len(f) == 2 {
+		// Remove whole comment.
+		line.Suffix = nil
+		return
+	}
+
+	// Remove comment prefix.
+	com := &line.Suffix[0]
+	i := strings.Index(com.Token, "indirect;")
+	com.Token = "//" + com.Token[i+len("indirect;"):]
+}
+
+// IsDirectoryPath reports whether the given path should be interpreted
+// as a directory path. Just like on the go command line, relative paths
+// and rooted paths are directory paths; the rest are module paths.
+func IsDirectoryPath(ns string) bool {
+	// Because go.mod files can move from one system to another,
+	// we check all known path syntaxes, both Unix and Windows.
+	return strings.HasPrefix(ns, "./") || strings.HasPrefix(ns, "../") || strings.HasPrefix(ns, "/") ||
+		strings.HasPrefix(ns, `.\`) || strings.HasPrefix(ns, `..\`) || strings.HasPrefix(ns, `\`) ||
+		len(ns) >= 2 && ('A' <= ns[0] && ns[0] <= 'Z' || 'a' <= ns[0] && ns[0] <= 'z') && ns[1] == ':'
+}
+
+// MustQuote reports whether s must be quoted in order to appear as
+// a single token in a go.mod line.
+func MustQuote(s string) bool {
+	for _, r := range s {
+		if !unicode.IsPrint(r) || r == ' ' || r == '"' || r == '\'' || r == '`' {
+			return true
+		}
+	}
+	return s == "" || strings.Contains(s, "//") || strings.Contains(s, "/*")
+}
+
+// AutoQuote returns s or, if quoting is required for s to appear in a go.mod,
+// the quotation of s.
+func AutoQuote(s string) string {
+	if MustQuote(s) {
+		return strconv.Quote(s)
+	}
+	return s
+}
+
+func parseString(s *string) (string, error) {
+	t := *s
+	if strings.HasPrefix(t, `"`) {
+		var err error
+		if t, err = strconv.Unquote(t); err != nil {
+			return "", err
+		}
+	} else if strings.ContainsAny(t, "\"'`") {
+		// Other quotes are reserved both for possible future expansion
+		// and to avoid confusion. For example if someone types 'x'
+		// we want that to be a syntax error and not a literal x in literal quotation marks.
+		return "", fmt.Errorf("unquoted string cannot contain quote")
+	}
+	*s = AutoQuote(t)
+	return t, nil
+}
+
+type Error struct {
+	Verb    string
+	ModPath string
+	Err     error
+}
+
+func (e *Error) Error() string {
+	return fmt.Sprintf("%s %s: %v", e.Verb, e.ModPath, e.Err)
+}
+
+func (e *Error) Unwrap() error { return e.Err }
+
+func parseVersion(verb string, path string, s *string, fix VersionFixer) (string, error) {
+	t, err := parseString(s)
+	if err != nil {
+		return "", &Error{
+			Verb:    verb,
+			ModPath: path,
+			Err: &module.InvalidVersionError{
+				Version: *s,
+				Err:     err,
+			},
+		}
+	}
+	if fix != nil {
+		var err error
+		t, err = fix(path, t)
+		if err != nil {
+			if err, ok := err.(*module.ModuleError); ok {
+				return "", &Error{
+					Verb:    verb,
+					ModPath: path,
+					Err:     err.Err,
+				}
+			}
+			return "", err
+		}
+	}
+	if v := module.CanonicalVersion(t); v != "" {
+		*s = v
+		return *s, nil
+	}
+	return "", &Error{
+		Verb:    verb,
+		ModPath: path,
+		Err: &module.InvalidVersionError{
+			Version: t,
+			Err:     errors.New("must be of the form v1.2.3"),
+		},
+	}
+}
+
+func modulePathMajor(path string) (string, error) {
+	_, major, ok := module.SplitPathVersion(path)
+	if !ok {
+		return "", fmt.Errorf("invalid module path")
+	}
+	return major, nil
+}
+
+func (f *File) Format() ([]byte, error) {
+	return Format(f.Syntax), nil
+}
+
+// Cleanup cleans up the file f after any edit operations.
+// To avoid quadratic behavior, modifications like DropRequire
+// clear the entry but do not remove it from the slice.
+// Cleanup cleans out all the cleared entries.
+func (f *File) Cleanup() {
+	w := 0
+	for _, r := range f.Require {
+		if r.Mod.Path != "" {
+			f.Require[w] = r
+			w++
+		}
+	}
+	f.Require = f.Require[:w]
+
+	w = 0
+	for _, x := range f.Exclude {
+		if x.Mod.Path != "" {
+			f.Exclude[w] = x
+			w++
+		}
+	}
+	f.Exclude = f.Exclude[:w]
+
+	w = 0
+	for _, r := range f.Replace {
+		if r.Old.Path != "" {
+			f.Replace[w] = r
+			w++
+		}
+	}
+	f.Replace = f.Replace[:w]
+
+	f.Syntax.Cleanup()
+}
+
+func (f *File) AddGoStmt(version string) error {
+	if !GoVersionRE.MatchString(version) {
+		return fmt.Errorf("invalid language version string %q", version)
+	}
+	if f.Go == nil {
+		f.Go = &Go{
+			Version: version,
+			Syntax:  f.Syntax.addLine(nil, "go", version),
+		}
+	} else {
+		f.Go.Version = version
+		f.Syntax.updateLine(f.Go.Syntax, "go", version)
+	}
+	return nil
+}
+
+func (f *File) AddRequire(path, vers string) error {
+	need := true
+	for _, r := range f.Require {
+		if r.Mod.Path == path {
+			if need {
+				r.Mod.Version = vers
+				f.Syntax.updateLine(r.Syntax, "require", AutoQuote(path), vers)
+				need = false
+			} else {
+				f.Syntax.removeLine(r.Syntax)
+				*r = Require{}
+			}
+		}
+	}
+
+	if need {
+		f.AddNewRequire(path, vers, false)
+	}
+	return nil
+}
+
+func (f *File) AddNewRequire(path, vers string, indirect bool) {
+	line := f.Syntax.addLine(nil, "require", AutoQuote(path), vers)
+	setIndirect(line, indirect)
+	f.Require = append(f.Require, &Require{module.Version{Path: path, Version: vers}, indirect, line})
+}
+
+func (f *File) SetRequire(req []*Require) {
+	need := make(map[string]string)
+	indirect := make(map[string]bool)
+	for _, r := range req {
+		need[r.Mod.Path] = r.Mod.Version
+		indirect[r.Mod.Path] = r.Indirect
+	}
+
+	for _, r := range f.Require {
+		if v, ok := need[r.Mod.Path]; ok {
+			r.Mod.Version = v
+			r.Indirect = indirect[r.Mod.Path]
+		}
+	}
+
+	var newStmts []Expr
+	for _, stmt := range f.Syntax.Stmt {
+		switch stmt := stmt.(type) {
+		case *LineBlock:
+			if len(stmt.Token) > 0 && stmt.Token[0] == "require" {
+				var newLines []*Line
+				for _, line := range stmt.Line {
+					if p, err := parseString(&line.Token[0]); err == nil && need[p] != "" {
+						if len(line.Comments.Before) == 1 && len(line.Comments.Before[0].Token) == 0 {
+							line.Comments.Before = line.Comments.Before[:0]
+						}
+						line.Token[1] = need[p]
+						delete(need, p)
+						setIndirect(line, indirect[p])
+						newLines = append(newLines, line)
+					}
+				}
+				if len(newLines) == 0 {
+					continue // drop stmt
+				}
+				stmt.Line = newLines
+			}
+
+		case *Line:
+			if len(stmt.Token) > 0 && stmt.Token[0] == "require" {
+				if p, err := parseString(&stmt.Token[1]); err == nil && need[p] != "" {
+					stmt.Token[2] = need[p]
+					delete(need, p)
+					setIndirect(stmt, indirect[p])
+				} else {
+					continue // drop stmt
+				}
+			}
+		}
+		newStmts = append(newStmts, stmt)
+	}
+	f.Syntax.Stmt = newStmts
+
+	for path, vers := range need {
+		f.AddNewRequire(path, vers, indirect[path])
+	}
+	f.SortBlocks()
+}
+
+func (f *File) DropRequire(path string) error {
+	for _, r := range f.Require {
+		if r.Mod.Path == path {
+			f.Syntax.removeLine(r.Syntax)
+			*r = Require{}
+		}
+	}
+	return nil
+}
+
+func (f *File) AddExclude(path, vers string) error {
+	var hint *Line
+	for _, x := range f.Exclude {
+		if x.Mod.Path == path && x.Mod.Version == vers {
+			return nil
+		}
+		if x.Mod.Path == path {
+			hint = x.Syntax
+		}
+	}
+
+	f.Exclude = append(f.Exclude, &Exclude{Mod: module.Version{Path: path, Version: vers}, Syntax: f.Syntax.addLine(hint, "exclude", AutoQuote(path), vers)})
+	return nil
+}
+
+func (f *File) DropExclude(path, vers string) error {
+	for _, x := range f.Exclude {
+		if x.Mod.Path == path && x.Mod.Version == vers {
+			f.Syntax.removeLine(x.Syntax)
+			*x = Exclude{}
+		}
+	}
+	return nil
+}
+
+func (f *File) AddReplace(oldPath, oldVers, newPath, newVers string) error {
+	need := true
+	old := module.Version{Path: oldPath, Version: oldVers}
+	new := module.Version{Path: newPath, Version: newVers}
+	tokens := []string{"replace", AutoQuote(oldPath)}
+	if oldVers != "" {
+		tokens = append(tokens, oldVers)
+	}
+	tokens = append(tokens, "=>", AutoQuote(newPath))
+	if newVers != "" {
+		tokens = append(tokens, newVers)
+	}
+
+	var hint *Line
+	for _, r := range f.Replace {
+		if r.Old.Path == oldPath && (oldVers == "" || r.Old.Version == oldVers) {
+			if need {
+				// Found replacement for old; update to use new.
+				r.New = new
+				f.Syntax.updateLine(r.Syntax, tokens...)
+				need = false
+				continue
+			}
+			// Already added; delete other replacements for same.
+			f.Syntax.removeLine(r.Syntax)
+			*r = Replace{}
+		}
+		if r.Old.Path == oldPath {
+			hint = r.Syntax
+		}
+	}
+	if need {
+		f.Replace = append(f.Replace, &Replace{Old: old, New: new, Syntax: f.Syntax.addLine(hint, tokens...)})
+	}
+	return nil
+}
+
+func (f *File) DropReplace(oldPath, oldVers string) error {
+	for _, r := range f.Replace {
+		if r.Old.Path == oldPath && r.Old.Version == oldVers {
+			f.Syntax.removeLine(r.Syntax)
+			*r = Replace{}
+		}
+	}
+	return nil
+}
+
+func (f *File) SortBlocks() {
+	f.removeDups() // otherwise sorting is unsafe
+
+	for _, stmt := range f.Syntax.Stmt {
+		block, ok := stmt.(*LineBlock)
+		if !ok {
+			continue
+		}
+		sort.Slice(block.Line, func(i, j int) bool {
+			li := block.Line[i]
+			lj := block.Line[j]
+			for k := 0; k < len(li.Token) && k < len(lj.Token); k++ {
+				if li.Token[k] != lj.Token[k] {
+					return li.Token[k] < lj.Token[k]
+				}
+			}
+			return len(li.Token) < len(lj.Token)
+		})
+	}
+}
+
+func (f *File) removeDups() {
+	have := make(map[module.Version]bool)
+	kill := make(map[*Line]bool)
+	for _, x := range f.Exclude {
+		if have[x.Mod] {
+			kill[x.Syntax] = true
+			continue
+		}
+		have[x.Mod] = true
+	}
+	var excl []*Exclude
+	for _, x := range f.Exclude {
+		if !kill[x.Syntax] {
+			excl = append(excl, x)
+		}
+	}
+	f.Exclude = excl
+
+	have = make(map[module.Version]bool)
+	// Later replacements take priority over earlier ones.
+	for i := len(f.Replace) - 1; i >= 0; i-- {
+		x := f.Replace[i]
+		if have[x.Old] {
+			kill[x.Syntax] = true
+			continue
+		}
+		have[x.Old] = true
+	}
+	var repl []*Replace
+	for _, x := range f.Replace {
+		if !kill[x.Syntax] {
+			repl = append(repl, x)
+		}
+	}
+	f.Replace = repl
+
+	var stmts []Expr
+	for _, stmt := range f.Syntax.Stmt {
+		switch stmt := stmt.(type) {
+		case *Line:
+			if kill[stmt] {
+				continue
+			}
+		case *LineBlock:
+			var lines []*Line
+			for _, line := range stmt.Line {
+				if !kill[line] {
+					lines = append(lines, line)
+				}
+			}
+			stmt.Line = lines
+			if len(lines) == 0 {
+				continue
+			}
+		}
+		stmts = append(stmts, stmt)
+	}
+	f.Syntax.Stmt = stmts
+}
diff --git a/modfile/rule_test.go b/modfile/rule_test.go
new file mode 100644
index 0000000..73e3386
--- /dev/null
+++ b/modfile/rule_test.go
@@ -0,0 +1,163 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfile
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"golang.org/x/mod/module"
+)
+
+var addRequireTests = []struct {
+	in   string
+	path string
+	vers string
+	out  string
+}{
+	{
+		`
+		module m
+		require x.y/z v1.2.3
+		`,
+		"x.y/z", "v1.5.6",
+		`
+		module m
+		require x.y/z v1.5.6
+		`,
+	},
+	{
+		`
+		module m
+		require x.y/z v1.2.3
+		`,
+		"x.y/w", "v1.5.6",
+		`
+		module m
+		require (
+			x.y/z v1.2.3
+			x.y/w v1.5.6
+		)
+		`,
+	},
+	{
+		`
+		module m
+		require x.y/z v1.2.3
+		require x.y/q/v2 v2.3.4
+		`,
+		"x.y/w", "v1.5.6",
+		`
+		module m
+		require x.y/z v1.2.3
+		require (
+			x.y/q/v2 v2.3.4
+			x.y/w v1.5.6
+		)
+		`,
+	},
+}
+
+var setRequireTests = []struct {
+	in   string
+	mods []struct {
+		path string
+		vers string
+	}
+	out string
+}{
+	{
+		`module m
+		require (
+			x.y/b v1.2.3
+
+			x.y/a v1.2.3
+		)
+		`,
+		[]struct {
+			path string
+			vers string
+		}{
+			{"x.y/a", "v1.2.3"},
+			{"x.y/b", "v1.2.3"},
+			{"x.y/c", "v1.2.3"},
+		},
+		`module m
+		require (
+			x.y/a v1.2.3
+			x.y/b v1.2.3
+			x.y/c v1.2.3
+		)
+		`,
+	},
+}
+
+func TestAddRequire(t *testing.T) {
+	for i, tt := range addRequireTests {
+		t.Run(fmt.Sprintf("#%d", i), func(t *testing.T) {
+			f, err := Parse("in", []byte(tt.in), nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			g, err := Parse("out", []byte(tt.out), nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			golden, err := g.Format()
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if err := f.AddRequire(tt.path, tt.vers); err != nil {
+				t.Fatal(err)
+			}
+			out, err := f.Format()
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !bytes.Equal(out, golden) {
+				t.Errorf("have:\n%s\nwant:\n%s", out, golden)
+			}
+		})
+	}
+}
+
+func TestSetRequire(t *testing.T) {
+	for i, tt := range setRequireTests {
+		t.Run(fmt.Sprintf("#%d", i), func(t *testing.T) {
+			f, err := Parse("in", []byte(tt.in), nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			g, err := Parse("out", []byte(tt.out), nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			golden, err := g.Format()
+			if err != nil {
+				t.Fatal(err)
+			}
+			var mods []*Require
+			for _, mod := range tt.mods {
+				mods = append(mods, &Require{
+					Mod: module.Version{
+						Path:    mod.path,
+						Version: mod.vers,
+					},
+				})
+			}
+
+			f.SetRequire(mods)
+			out, err := f.Format()
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !bytes.Equal(out, golden) {
+				t.Errorf("have:\n%s\nwant:\n%s", out, golden)
+			}
+		})
+	}
+}
diff --git a/modfile/testdata/block.golden b/modfile/testdata/block.golden
new file mode 100644
index 0000000..4aa2d63
--- /dev/null
+++ b/modfile/testdata/block.golden
@@ -0,0 +1,29 @@
+// comment
+x "y" z
+
+// block
+block ( // block-eol
+	// x-before-line
+
+	"x" ( y // x-eol
+	"x1"
+	"x2"
+	// line
+	"x3"
+	"x4"
+
+	"x5"
+
+	// y-line
+	"y" //  y-eol
+
+	"z" // z-eol
+) // block-eol2
+
+block2 (
+	x
+	y
+	z
+)
+
+// eof
diff --git a/modfile/testdata/block.in b/modfile/testdata/block.in
new file mode 100644
index 0000000..1dfae65
--- /dev/null
+++ b/modfile/testdata/block.in
@@ -0,0 +1,29 @@
+// comment
+x "y" z
+
+// block
+block ( // block-eol
+	// x-before-line
+	
+	"x" ( y // x-eol
+	"x1"
+	"x2"
+	// line
+	"x3"
+	"x4"
+	
+	"x5"
+	
+	// y-line
+	"y" //  y-eol
+	
+	"z" // z-eol
+) // block-eol2
+
+
+block2 (x
+	y
+	z
+)
+
+// eof
diff --git a/modfile/testdata/comment.golden b/modfile/testdata/comment.golden
new file mode 100644
index 0000000..75f3b84
--- /dev/null
+++ b/modfile/testdata/comment.golden
@@ -0,0 +1,10 @@
+// comment
+module "x" // eol
+
+// mid comment
+
+// comment 2
+// comment 2 line 2
+module "y" // eoy
+
+// comment 3
diff --git a/modfile/testdata/comment.in b/modfile/testdata/comment.in
new file mode 100644
index 0000000..bfc2492
--- /dev/null
+++ b/modfile/testdata/comment.in
@@ -0,0 +1,8 @@
+// comment
+module "x" // eol
+// mid comment
+
+// comment 2
+// comment 2 line 2
+module "y" // eoy
+// comment 3
diff --git a/modfile/testdata/empty.golden b/modfile/testdata/empty.golden
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/modfile/testdata/empty.golden
diff --git a/modfile/testdata/empty.in b/modfile/testdata/empty.in
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/modfile/testdata/empty.in
diff --git a/modfile/testdata/gopkg.in.golden b/modfile/testdata/gopkg.in.golden
new file mode 100644
index 0000000..41669b3
--- /dev/null
+++ b/modfile/testdata/gopkg.in.golden
@@ -0,0 +1,6 @@
+module x
+
+require (
+	gopkg.in/mgo.v2 v2.0.0-20160818020120-3f83fa500528
+	gopkg.in/yaml.v2 v2.2.1
+)
diff --git a/modfile/testdata/module.golden b/modfile/testdata/module.golden
new file mode 100644
index 0000000..78ba943
--- /dev/null
+++ b/modfile/testdata/module.golden
@@ -0,0 +1 @@
+module abc
diff --git a/modfile/testdata/module.in b/modfile/testdata/module.in
new file mode 100644
index 0000000..08f3836
--- /dev/null
+++ b/modfile/testdata/module.in
@@ -0,0 +1 @@
+module "abc"
diff --git a/modfile/testdata/replace.golden b/modfile/testdata/replace.golden
new file mode 100644
index 0000000..5d6abcf
--- /dev/null
+++ b/modfile/testdata/replace.golden
@@ -0,0 +1,5 @@
+module abc
+
+replace xyz v1.2.3 => /tmp/z
+
+replace xyz v1.3.4 => my/xyz v1.3.4-me
diff --git a/modfile/testdata/replace.in b/modfile/testdata/replace.in
new file mode 100644
index 0000000..6852499
--- /dev/null
+++ b/modfile/testdata/replace.in
@@ -0,0 +1,5 @@
+module "abc"
+
+replace "xyz" v1.2.3 => "/tmp/z"
+
+replace "xyz" v1.3.4 => "my/xyz" v1.3.4-me
diff --git a/modfile/testdata/replace2.golden b/modfile/testdata/replace2.golden
new file mode 100644
index 0000000..e1d9c72
--- /dev/null
+++ b/modfile/testdata/replace2.golden
@@ -0,0 +1,10 @@
+module abc
+
+replace (
+	xyz v1.2.3 => /tmp/z
+	xyz v1.3.4 => my/xyz v1.3.4-me
+	xyz v1.4.5 => "/tmp/my dir"
+	xyz v1.5.6 => my/xyz v1.5.6
+
+	xyz => my/other/xyz v1.5.4
+)
diff --git a/modfile/testdata/replace2.in b/modfile/testdata/replace2.in
new file mode 100644
index 0000000..7864698
--- /dev/null
+++ b/modfile/testdata/replace2.in
@@ -0,0 +1,10 @@
+module "abc"
+
+replace (
+	"xyz" v1.2.3 => "/tmp/z"
+	"xyz" v1.3.4 => "my/xyz" "v1.3.4-me"
+	xyz "v1.4.5" => "/tmp/my dir"
+	xyz v1.5.6 => my/xyz v1.5.6
+
+	xyz => my/other/xyz v1.5.4
+)
diff --git a/modfile/testdata/rule1.golden b/modfile/testdata/rule1.golden
new file mode 100644
index 0000000..8a5c725
--- /dev/null
+++ b/modfile/testdata/rule1.golden
@@ -0,0 +1,7 @@
+module "x"
+
+module "y"
+
+require "x"
+
+require x