diff --git a/src/cmd/gofmt/Makefile b/src/cmd/gofmt/Makefile
index a93b8c3..dbc134f 100644
--- a/src/cmd/gofmt/Makefile
+++ b/src/cmd/gofmt/Makefile
@@ -7,6 +7,7 @@
 TARG=gofmt
 GOFILES=\
 	gofmt.go\
+	rewrite.go\
 
 include $(GOROOT)/src/Make.cmd
 
diff --git a/src/cmd/gofmt/gofmt.go b/src/cmd/gofmt/gofmt.go
index bec4c88..d7c96dc 100644
--- a/src/cmd/gofmt/gofmt.go
+++ b/src/cmd/gofmt/gofmt.go
@@ -8,6 +8,7 @@
 	"bytes";
 	"flag";
 	"fmt";
+	"go/ast";
 	"go/parser";
 	"go/printer";
 	"go/scanner";
@@ -20,8 +21,9 @@
 
 var (
 	// main operation modes
-	list	= flag.Bool("l", false, "list files whose formatting differs from gofmt's");
-	write	= flag.Bool("w", false, "write result to (source) file instead of stdout");
+	list		= flag.Bool("l", false, "list files whose formatting differs from gofmt's");
+	write		= flag.Bool("w", false, "write result to (source) file instead of stdout");
+	rewriteRule	= flag.String("r", "", "rewrite rule (e.g., 'α[β:len(α)] -> α[β:]')");
 
 	// debugging support
 	comments	= flag.Bool("comments", true, "print comments");
@@ -34,6 +36,8 @@
 
 
 var exitCode = 0
+var rewrite func(*ast.File) *ast.File
+
 
 func report(err os.Error) {
 	scanner.PrintError(os.Stderr, err);
@@ -86,6 +90,10 @@
 		return err
 	}
 
+	if rewrite != nil {
+		file = rewrite(file)
+	}
+
 	var res bytes.Buffer;
 	_, err = (&printer.Config{printerMode(), *tabwidth, nil}).Fprint(&res, file);
 	if err != nil {
@@ -154,6 +162,8 @@
 		os.Exit(2);
 	}
 
+	initRewrite();
+
 	if flag.NArg() == 0 {
 		if err := processFile("/dev/stdin"); err != nil {
 			report(err)
diff --git a/src/cmd/gofmt/rewrite.go b/src/cmd/gofmt/rewrite.go
new file mode 100644
index 0000000..9399bcd
--- /dev/null
+++ b/src/cmd/gofmt/rewrite.go
@@ -0,0 +1,226 @@
+// Copyright 2009 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 main
+
+import (
+	"fmt";
+	"go/ast";
+	"go/parser";
+	"go/token";
+	"os";
+	"reflect";
+	"strings";
+	"unicode";
+	"utf8";
+)
+
+
+func initRewrite() {
+	if *rewriteRule == "" {
+		return
+	}
+	f := strings.Split(*rewriteRule, "->", 0);
+	if len(f) != 2 {
+		fmt.Fprintf(os.Stderr, "rewrite rule must be of the form 'pattern -> replacement'\n");
+		os.Exit(2);
+	}
+	pattern := parseExpr(f[0], "pattern");
+	replace := parseExpr(f[1], "replacement");
+	rewrite = func(p *ast.File) *ast.File { return rewriteFile(pattern, replace, p) };
+}
+
+
+// parseExpr parses s as an expression.
+// It might make sense to expand this to allow statement patterns,
+// but there are problems with preserving formatting and also
+// with what a wildcard for a statement looks like.
+func parseExpr(s string, what string) ast.Expr {
+	stmts, err := parser.ParseStmtList("input", s);
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "parsing %s %s: %s\n", what, s, err);
+		os.Exit(2);
+	}
+	if len(stmts) != 1 {
+		fmt.Fprintf(os.Stderr, "%s must be single expression\n", what);
+		os.Exit(2);
+	}
+	x, ok := stmts[0].(*ast.ExprStmt);
+	if !ok {
+		fmt.Fprintf(os.Stderr, "%s must be single expression\n", what);
+		os.Exit(2);
+	}
+	return x.X;
+}
+
+
+// rewriteFile applys the rewrite rule pattern -> replace to an entire file.
+func rewriteFile(pattern, replace ast.Expr, p *ast.File) *ast.File {
+	m := make(map[string]reflect.Value);
+	pat := reflect.NewValue(pattern);
+	repl := reflect.NewValue(replace);
+	var f func(val reflect.Value) reflect.Value;	// f is recursive
+	f = func(val reflect.Value) reflect.Value {
+		for k := range m {
+			m[k] = nil, false
+		}
+		if match(m, pat, val) {
+			return subst(m, repl)
+		}
+		return apply(f, val);
+	};
+	return apply(f, reflect.NewValue(p)).Interface().(*ast.File);
+}
+
+
+var positionType = reflect.Typeof(token.Position{})
+var zeroPosition = reflect.NewValue(token.Position{})
+var identType = reflect.Typeof((*ast.Ident)(nil))
+
+
+func isWildcard(s string) bool {
+	rune, _ := utf8.DecodeRuneInString(s);
+	return unicode.Is(unicode.Greek, rune) && unicode.IsLower(rune);
+}
+
+
+// apply replaces each AST field x in val with f(x), returning val.
+// To avoid extra conversions, f operates on the reflect.Value form.
+func apply(f func(reflect.Value) reflect.Value, val reflect.Value) reflect.Value {
+	if val == nil {
+		return nil
+	}
+	switch v := reflect.Indirect(val).(type) {
+	case *reflect.SliceValue:
+		for i := 0; i < v.Len(); i++ {
+			e := v.Elem(i);
+			e.SetValue(f(e));
+		}
+	case *reflect.StructValue:
+		for i := 0; i < v.NumField(); i++ {
+			e := v.Field(i);
+			e.SetValue(f(e));
+		}
+	case *reflect.InterfaceValue:
+		e := v.Elem();
+		v.SetValue(f(e));
+	}
+	return val;
+}
+
+
+// match returns true if pattern matches val,
+// recording wildcard submatches in m.
+// If m == nil, match checks whether pattern == val.
+func match(m map[string]reflect.Value, pattern, val reflect.Value) bool {
+	// Wildcard matches any expression.  If it appears multiple
+	// times in the pattern, it must match the same expression
+	// each time.
+	if m != nil && pattern.Type() == identType {
+		name := pattern.Interface().(*ast.Ident).Value;
+		if isWildcard(name) {
+			if old, ok := m[name]; ok {
+				return match(nil, old, val)
+			}
+			m[name] = val;
+			return true;
+		}
+	}
+
+	// Otherwise, the expressions must match recursively.
+	if pattern == nil || val == nil {
+		return pattern == nil && val == nil
+	}
+	if pattern.Type() != val.Type() {
+		return false
+	}
+
+	// Token positions need not match.
+	if pattern.Type() == positionType {
+		return true
+	}
+
+	p := reflect.Indirect(pattern);
+	v := reflect.Indirect(val);
+
+	switch p := p.(type) {
+	case *reflect.SliceValue:
+		v := v.(*reflect.SliceValue);
+		for i := 0; i < p.Len(); i++ {
+			if !match(m, p.Elem(i), v.Elem(i)) {
+				return false
+			}
+		}
+		return true;
+
+	case *reflect.StructValue:
+		v := v.(*reflect.StructValue);
+		for i := 0; i < p.NumField(); i++ {
+			if !match(m, p.Field(i), v.Field(i)) {
+				return false
+			}
+		}
+		return true;
+
+	case *reflect.InterfaceValue:
+		v := v.(*reflect.InterfaceValue);
+		return match(m, p.Elem(), v.Elem());
+	}
+
+	// Handle token integers, etc.
+	return p.Interface() == v.Interface();
+}
+
+
+// subst returns a copy of pattern with values from m substituted in place of wildcards.
+// if m == nil, subst returns a copy of pattern.
+// Either way, the returned value has no valid line number information.
+func subst(m map[string]reflect.Value, pattern reflect.Value) reflect.Value {
+	if pattern == nil {
+		return nil
+	}
+
+	// Wildcard gets replaced with map value.
+	if m != nil && pattern.Type() == identType {
+		name := pattern.Interface().(*ast.Ident).Value;
+		if isWildcard(name) {
+			if old, ok := m[name]; ok {
+				return subst(nil, old)
+			}
+		}
+	}
+
+	if pattern.Type() == positionType {
+		return zeroPosition
+	}
+
+	// Otherwise copy.
+	switch p := pattern.(type) {
+	case *reflect.SliceValue:
+		v := reflect.MakeSlice(p.Type().(*reflect.SliceType), p.Len(), p.Len());
+		for i := 0; i < p.Len(); i++ {
+			v.Elem(i).SetValue(subst(m, p.Elem(i)))
+		}
+		return v;
+
+	case *reflect.StructValue:
+		v := reflect.MakeZero(p.Type()).(*reflect.StructValue);
+		for i := 0; i < p.NumField(); i++ {
+			v.Field(i).SetValue(subst(m, p.Field(i)))
+		}
+		return v;
+
+	case *reflect.PtrValue:
+		v := reflect.MakeZero(p.Type()).(*reflect.PtrValue);
+		v.PointTo(subst(m, p.Elem()));
+		return v;
+
+	case *reflect.InterfaceValue:
+		v := reflect.MakeZero(p.Type()).(*reflect.InterfaceValue);
+		v.SetValue(subst(m, p.Elem()));
+		return v;
+	}
+
+	return pattern;
+}
diff --git a/src/pkg/go/printer/nodes.go b/src/pkg/go/printer/nodes.go
index 6304830..1c74603 100644
--- a/src/pkg/go/printer/nodes.go
+++ b/src/pkg/go/printer/nodes.go
@@ -52,6 +52,17 @@
 	case n > max:
 		n = max
 	}
+
+	// TODO(gri): try to avoid direct manipulation of p.pos
+	// demo of why this is necessary: run gofmt -r 'i < i -> i < j' x.go on this x.go:
+	//	package main
+	//	func main() {
+	//		i < i;
+	//		j < 10;
+	//	}
+	//
+	p.pos.Line += n;
+
 	if n > 0 {
 		p.print(ws);
 		if newSection {
@@ -199,7 +210,7 @@
 			if mode&commaSep != 0 {
 				p.print(token.COMMA)
 			}
-			if prev < line {
+			if prev < line && prev > 0 && line > 0 {
 				if p.linebreak(line, 1, 2, ws, true) {
 					ws = ignore;
 					*multiLine = true;
@@ -564,8 +575,7 @@
 	xline := p.pos.Line;	// before the operator (it may be on the next line!)
 	yline := x.Y.Pos().Line;
 	p.print(x.OpPos, x.Op);
-	if xline != yline {
-		//println(x.OpPos.String());
+	if xline != yline && xline > 0 && yline > 0 {
 		// at least one line break, but respect an extra empty line
 		// in the source
 		if p.linebreak(yline, 1, 2, ws, true) {