diff --git a/runes/cond.go b/runes/cond.go
new file mode 100644
index 0000000..1a97872
--- /dev/null
+++ b/runes/cond.go
@@ -0,0 +1,126 @@
+// Copyright 2015 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 runes
+
+import (
+	"unicode/utf8"
+
+	"golang.org/x/text/transform"
+)
+
+// Note: below we pass invalid UTF-8 to the tIn and tNotIn transformers as is.
+// This is done for various reasons:
+// - To retain the semantics of the Nop transformer: if input is passed to a Nop
+//   one would expect it to be unchanged.
+// - It would be very expensive to pass a converted RuneError to a transformer:
+//   a transformer might need more source bytes after RuneError, meaning that
+//   the only way to pass it safely is to create a new buffer and manage the
+//   intermingling of RuneErrors and normal input.
+// - Many transformers leave ill-formed UTF-8 as is, so this is not
+//   inconsistent. Generally ill-formed UTF-8 is only replaced if it is a
+//   logical consequence of the operation (as for Map) or if it otherwise would
+//   pose security concerns (as for Remove).
+// - An alternative would be to return an error on ill-formed UTF-8, but this
+//   would be inconsistent with other operations.
+
+// If returns a transformer that applies tIn to consecutive runes for which
+// s.Contains(r) and tNotIn to consecutive runes for which !s.Contains(r). Reset
+// is called on tIn and tNotIn at the start of each run. A Nop transformer will
+// substitute a nil value passed to tIn or tNotIn. Invalid UTF-8 is translated
+// to RuneError to determine which transformer to apply, but is passed as is to
+// the respective transformer.
+func If(s Set, tIn, tNotIn transform.Transformer) transform.Transformer {
+	if tIn == nil && tNotIn == nil {
+		return transform.Nop
+	}
+	if tIn == nil {
+		tIn = transform.Nop
+	}
+	if tNotIn == nil {
+		tNotIn = transform.Nop
+	}
+	a := &cond{
+		tIn:    tIn,
+		tNotIn: tNotIn,
+		f:      s.Contains,
+	}
+	a.Reset()
+	return a
+}
+
+type cond struct {
+	tIn, tNotIn transform.Transformer
+	f           func(rune) bool
+	check       func(rune) bool       // current check to perform
+	t           transform.Transformer // current transformer to use
+}
+
+// Reset implements transform.Transformer.
+func (t *cond) Reset() {
+	t.check = t.is
+	t.t = t.tIn
+	t.t.Reset() // notIn will be reset on first usage.
+}
+
+func (t *cond) is(r rune) bool {
+	if t.f(r) {
+		return true
+	}
+	t.check = t.isNot
+	t.t = t.tNotIn
+	t.tNotIn.Reset()
+	return false
+}
+
+func (t *cond) isNot(r rune) bool {
+	if !t.f(r) {
+		return true
+	}
+	t.check = t.is
+	t.t = t.tIn
+	t.tIn.Reset()
+	return false
+}
+
+func (t *cond) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
+	p := 0
+	for nSrc < len(src) && err == nil {
+		// Don't process too much at a time, as the work might be wasted if the
+		// destination buffer isn't large enough to hold the result or a
+		// transform returns an error early.
+		const maxChunk = 4096
+		max := len(src)
+		if n := nSrc + maxChunk; n < len(src) {
+			max = n
+		}
+		atEnd := false
+		size := 0
+		current := t.t
+		for ; p < max; p += size {
+			var r rune
+			r, size = utf8.DecodeRune(src[p:])
+			if r == utf8.RuneError && size == 1 {
+				if !atEOF && !utf8.FullRune(src[p:]) {
+					err = transform.ErrShortSrc
+					break
+				}
+			}
+			if !t.check(r) {
+				// The next rune will be the start of a new run.
+				atEnd = true
+				break
+			}
+		}
+		nDst2, nSrc2, err2 := current.Transform(dst[nDst:], src[nSrc:p], atEnd || (atEOF && p == len(src)))
+		nDst += nDst2
+		nSrc += nSrc2
+		if err2 != nil {
+			return nDst, nSrc, err2
+		}
+		// At this point either err != nil or t.check will pass for the rune at p.
+		p = nSrc + size
+	}
+	return nDst, nSrc, err
+}
diff --git a/runes/cond_test.go b/runes/cond_test.go
new file mode 100644
index 0000000..175bd2b
--- /dev/null
+++ b/runes/cond_test.go
@@ -0,0 +1,233 @@
+// Copyright 2015 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 runes
+
+import (
+	"strings"
+	"testing"
+	"unicode"
+
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
+	"golang.org/x/text/transform"
+)
+
+var (
+	toUpper = cases.Upper(language.Und)
+	toLower = cases.Lower(language.Und)
+)
+
+func TestPredicate(t *testing.T) {
+	testConditional(t, func(rt *unicode.RangeTable, t, f transform.Transformer) transform.Transformer {
+		return If(Predicate(func(r rune) bool {
+			return unicode.Is(rt, r)
+		}), t, f)
+	})
+}
+
+func TestIn(t *testing.T) {
+	testConditional(t, func(rt *unicode.RangeTable, t, f transform.Transformer) transform.Transformer {
+		return If(In(rt), t, f)
+	})
+}
+
+func TestNotIn(t *testing.T) {
+	testConditional(t, func(rt *unicode.RangeTable, t, f transform.Transformer) transform.Transformer {
+		return If(NotIn(rt), f, t)
+	})
+}
+
+func testConditional(t *testing.T, f func(rt *unicode.RangeTable, t, f transform.Transformer) transform.Transformer) {
+	lower := f(unicode.Latin, toLower, toLower)
+
+	for i, tt := range []transformTest{{
+		desc:    "empty",
+		szDst:   large,
+		atEOF:   true,
+		in:      "",
+		out:     "",
+		outFull: "",
+		t:       lower,
+	}, {
+		desc:    "small",
+		szDst:   1,
+		atEOF:   true,
+		in:      "B",
+		out:     "b",
+		outFull: "b",
+		t:       lower,
+	}, {
+		desc:    "short dst",
+		szDst:   2,
+		atEOF:   true,
+		in:      "AAA",
+		out:     "aa",
+		outFull: "aaa",
+		err:     transform.ErrShortDst,
+		t:       lower,
+	}, {
+		desc:    "short dst writing error",
+		szDst:   1,
+		atEOF:   false,
+		in:      "A\x80",
+		out:     "a",
+		outFull: "a\x80",
+		err:     transform.ErrShortDst,
+		t:       lower,
+	}, {
+		desc:    "short dst writing incomplete rune",
+		szDst:   2,
+		atEOF:   true,
+		in:      "Σ\xc0",
+		out:     "Σ",
+		outFull: "Σ\xc0",
+		err:     transform.ErrShortDst,
+		t:       f(unicode.Latin, toLower, nil),
+	}, {
+		desc:    "short dst, longer",
+		szDst:   5,
+		atEOF:   true,
+		in:      "Hellø",
+		out:     "Hell",
+		outFull: "Hellø",
+		err:     transform.ErrShortDst,
+		// idem is used to test short buffers by forcing processing of full-rune increments.
+		t: f(unicode.Latin, Map(idem), nil),
+	}, {
+		desc:    "short dst, longer, writing error",
+		szDst:   6,
+		atEOF:   false,
+		in:      "\x80Hello\x80",
+		out:     "\x80Hello",
+		outFull: "\x80Hello\x80",
+		err:     transform.ErrShortDst,
+		t:       f(unicode.Latin, Map(idem), nil),
+	}, {
+		desc:    "short src",
+		szDst:   2,
+		atEOF:   false,
+		in:      "A\xc0",
+		out:     "a",
+		outFull: "a\xc0",
+		err:     transform.ErrShortSrc,
+		t:       lower,
+	}, {
+		desc:    "invalid input, atEOF",
+		szDst:   large,
+		atEOF:   true,
+		in:      "\x80",
+		out:     "\x80",
+		outFull: "\x80",
+		t:       lower,
+	}, {
+		desc:    "invalid input, !atEOF",
+		szDst:   large,
+		atEOF:   false,
+		in:      "\x80",
+		out:     "\x80",
+		outFull: "\x80",
+		t:       lower,
+	}, {
+		desc:    "invalid input, incomplete rune atEOF",
+		szDst:   large,
+		atEOF:   true,
+		in:      "\xc0",
+		out:     "\xc0",
+		outFull: "\xc0",
+		t:       lower,
+	}, {
+		desc:    "nop",
+		szDst:   large,
+		atEOF:   true,
+		in:      "Hello World!",
+		out:     "Hello World!",
+		outFull: "Hello World!",
+		t:       f(unicode.Latin, nil, nil),
+	}, {
+		desc:    "nop in",
+		szDst:   large,
+		atEOF:   true,
+		in:      "THIS IS α ΤΕΣΤ",
+		out:     "this is α ΤΕΣΤ",
+		outFull: "this is α ΤΕΣΤ",
+		t:       f(unicode.Greek, nil, toLower),
+	}, {
+		desc:    "nop not in",
+		szDst:   large,
+		atEOF:   true,
+		in:      "THIS IS α ΤΕΣΤ",
+		out:     "this is α ΤΕΣΤ",
+		outFull: "this is α ΤΕΣΤ",
+		t:       f(unicode.Latin, toLower, nil),
+	}, {
+		desc:    "pass atEOF is true when at end",
+		szDst:   large,
+		atEOF:   true,
+		in:      "hello",
+		out:     "HELLO",
+		outFull: "HELLO",
+		t:       f(unicode.Latin, upperAtEOF{}, nil),
+	}, {
+		desc:    "pass atEOF is true when at end of segment",
+		szDst:   large,
+		atEOF:   true,
+		in:      "hello ",
+		out:     "HELLO ",
+		outFull: "HELLO ",
+		t:       f(unicode.Latin, upperAtEOF{}, nil),
+	}, {
+		desc:    "don't pass atEOF is true when atEOF is false",
+		szDst:   large,
+		atEOF:   false,
+		in:      "hello",
+		out:     "",
+		outFull: "HELLO",
+		t:       f(unicode.Latin, upperAtEOF{}, nil),
+		err:     transform.ErrShortSrc,
+	}, {
+		desc:    "large input ASCII",
+		szDst:   12000,
+		atEOF:   false,
+		in:      strings.Repeat("HELLO", 2000),
+		out:     strings.Repeat("hello", 2000),
+		outFull: strings.Repeat("hello", 2000),
+		t:       lower,
+		err:     nil,
+	}, {
+		desc:    "large input non-ASCII",
+		szDst:   12000,
+		atEOF:   false,
+		in:      strings.Repeat("\u3333", 2000),
+		out:     strings.Repeat("\u3333", 2000),
+		outFull: strings.Repeat("\u3333", 2000),
+		t:       lower,
+		err:     nil,
+	}} {
+		tt.check(t, i)
+	}
+}
+
+// upperAtEOF is a strange Transformer that converts text to uppercase, but only
+// if atEOF is true.
+type upperAtEOF struct{ transform.NopResetter }
+
+func (upperAtEOF) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
+	if !atEOF {
+		return 0, 0, transform.ErrShortSrc
+	}
+	return toUpper.Transform(dst, src, atEOF)
+}
+
+func BenchmarkConditional(b *testing.B) {
+	dst := make([]byte, len(input))
+	src := []byte(input)
+
+	r := If(In(unicode.Hangul), transform.Nop, transform.Nop)
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		r.Transform(dst, src, true)
+	}
+}
diff --git a/runes/example_test.go b/runes/example_test.go
new file mode 100644
index 0000000..a60bfd9
--- /dev/null
+++ b/runes/example_test.go
@@ -0,0 +1,60 @@
+// Copyright 2014 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 runes_test
+
+import (
+	"fmt"
+	"unicode"
+
+	"golang.org/x/text/runes"
+	"golang.org/x/text/transform"
+	"golang.org/x/text/unicode/norm"
+	"golang.org/x/text/width"
+)
+
+func ExampleRemove() {
+	t := transform.Chain(norm.NFD, runes.Remove(runes.In(unicode.Mn)), norm.NFC)
+	s, _, _ := transform.String(t, "résumé")
+	fmt.Println(s)
+
+	// Output:
+	// resume
+}
+
+func ExampleMap() {
+	replaceHyphens := runes.Map(func(r rune) rune {
+		if unicode.Is(unicode.Hyphen, r) {
+			return '|'
+		}
+		return r
+	})
+	s, _, _ := transform.String(replaceHyphens, "a-b‐c⸗d﹣e")
+	fmt.Println(s)
+
+	// Output:
+	// a|b|c|d|e
+}
+
+func ExampleIn() {
+	// Convert Latin characters to their canonical form, while keeping other
+	// width distinctions.
+	t := runes.If(runes.In(unicode.Latin), width.Fold, nil)
+	s, _, _ := transform.String(t, "ｱﾙｱﾉﾘｳ tech / アルアノリウ ｔｅｃｈ")
+	fmt.Println(s)
+
+	// Output:
+	// ｱﾙｱﾉﾘｳ tech / アルアノリウ tech
+}
+
+func ExampleIf() {
+	// Widen everything but ASCII.
+	isASCII := func(r rune) bool { return r <= unicode.MaxASCII }
+	t := runes.If(runes.Predicate(isASCII), nil, width.Widen)
+	s, _, _ := transform.String(t, "ｱﾙｱﾉﾘｳ tech / 中國 / 5₩")
+	fmt.Println(s)
+
+	// Output:
+	// アルアノリウ tech / 中國 / 5￦
+}
diff --git a/runes/runes.go b/runes/runes.go
new file mode 100644
index 0000000..6daa55b
--- /dev/null
+++ b/runes/runes.go
@@ -0,0 +1,196 @@
+// Copyright 2014 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 runes provide transforms for UTF-8 encoded text.
+package runes
+
+import (
+	"unicode"
+	"unicode/utf8"
+
+	"golang.org/x/text/transform"
+)
+
+// A Set is a collection of runes.
+type Set interface {
+	// Contains returns true if r is contained in the set.
+	Contains(r rune) bool
+}
+
+type setFunc func(rune) bool
+
+func (s setFunc) Contains(r rune) bool {
+	return s(r)
+}
+
+// Note: using funcs here instead of wrapping types result in cleaner
+// documentation and a smaller API.
+
+// In creates a Set with a Contains method that returns true for all runes in
+// the given RangeTable.
+func In(rt *unicode.RangeTable) Set {
+	return setFunc(func(r rune) bool { return unicode.Is(rt, r) })
+}
+
+// In creates a Set with a Contains method that returns true for all runes not
+// in the given RangeTable.
+func NotIn(rt *unicode.RangeTable) Set {
+	return setFunc(func(r rune) bool { return !unicode.Is(rt, r) })
+}
+
+// Predicate creates a Set with a Contains method that returns f(r).
+func Predicate(f func(rune) bool) Set {
+	return setFunc(f)
+}
+
+// TODO:
+// - Copy: copying strings and bytes in whole-rune units.
+// - Validation (maybe)
+// - Well-formed-ness (maybe)
+
+const runeErrorString = string(utf8.RuneError)
+
+// Remove returns a Transformer that removes runes r for which s.Contains(r).
+// Illegal input bytes are replaced by RuneError before being passed to f.
+func Remove(s Set) transform.Transformer {
+	if f, ok := s.(setFunc); ok {
+		// This little trick cuts the running time of BenchmarkRemove for sets
+		// created by Predicate roughly in half.
+		// TODO: special-case RangeTables as well.
+		return remove(f)
+	}
+	return remove(s.Contains)
+}
+
+// TODO: remove transform.RemoveFunc.
+
+type remove func(r rune) bool
+
+func (remove) Reset() {}
+
+// Transform implements transform.Transformer.
+func (t remove) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
+	for r, size := rune(0), 0; nSrc < len(src); {
+		if r = rune(src[nSrc]); r < utf8.RuneSelf {
+			size = 1
+		} else {
+			r, size = utf8.DecodeRune(src[nSrc:])
+
+			if size == 1 {
+				// Invalid rune.
+				if !atEOF && !utf8.FullRune(src[nSrc:]) {
+					err = transform.ErrShortSrc
+					break
+				}
+				// We replace illegal bytes with RuneError. Not doing so might
+				// otherwise turn a sequence of invalid UTF-8 into valid UTF-8.
+				// The resulting byte sequence may subsequently contain runes
+				// for which t(r) is true that were passed unnoticed.
+				if !t(utf8.RuneError) {
+					if nDst+3 > len(dst) {
+						err = transform.ErrShortDst
+						break
+					}
+					dst[nDst+0] = runeErrorString[0]
+					dst[nDst+1] = runeErrorString[1]
+					dst[nDst+2] = runeErrorString[2]
+					nDst += 3
+				}
+				nSrc++
+				continue
+			}
+		}
+
+		if t(r) {
+			nSrc += size
+			continue
+		}
+		if nDst+size > len(dst) {
+			err = transform.ErrShortDst
+			break
+		}
+		for i := 0; i < size; i++ {
+			dst[nDst] = src[nSrc]
+			nDst++
+			nSrc++
+		}
+	}
+	return
+}
+
+// Map returns a Transformer that maps the runes in the input using the given
+// mapping. Illegal bytes in the input are converted to utf8.RuneError before
+// being passed to the mapping func.
+func Map(mapping func(rune) rune) transform.Transformer {
+	return mapper(mapping)
+}
+
+type mapper func(rune) rune
+
+func (mapper) Reset() {}
+
+// Transform implements transform.Transformer.
+func (t mapper) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
+	var replacement rune
+	var b [utf8.UTFMax]byte
+
+	for r, size := rune(0), 0; nSrc < len(src); {
+		if r = rune(src[nSrc]); r < utf8.RuneSelf {
+			if replacement = t(r); replacement < utf8.RuneSelf {
+				if nDst == len(dst) {
+					err = transform.ErrShortDst
+					break
+				}
+				dst[nDst] = byte(replacement)
+				nDst++
+				nSrc++
+				continue
+			}
+			size = 1
+		} else if r, size = utf8.DecodeRune(src[nSrc:]); size == 1 {
+			// Invalid rune.
+			if !atEOF && !utf8.FullRune(src[nSrc:]) {
+				err = transform.ErrShortSrc
+				break
+			}
+
+			if replacement = t(utf8.RuneError); replacement == utf8.RuneError {
+				if nDst+3 > len(dst) {
+					err = transform.ErrShortDst
+					break
+				}
+				dst[nDst+0] = runeErrorString[0]
+				dst[nDst+1] = runeErrorString[1]
+				dst[nDst+2] = runeErrorString[2]
+				nDst += 3
+				nSrc++
+				continue
+			}
+		} else if replacement = t(r); replacement == r {
+			if nDst+size > len(dst) {
+				err = transform.ErrShortDst
+				break
+			}
+			for i := 0; i < size; i++ {
+				dst[nDst] = src[nSrc]
+				nDst++
+				nSrc++
+			}
+			continue
+		}
+
+		n := utf8.EncodeRune(b[:], replacement)
+
+		if nDst+n > len(dst) {
+			err = transform.ErrShortDst
+			break
+		}
+		for i := 0; i < n; i++ {
+			dst[nDst] = b[i]
+			nDst++
+		}
+		nSrc += size
+	}
+	return
+}
diff --git a/runes/runes_test.go b/runes/runes_test.go
new file mode 100644
index 0000000..4fbc333
--- /dev/null
+++ b/runes/runes_test.go
@@ -0,0 +1,451 @@
+// Copyright 2015 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 runes
+
+import (
+	"strings"
+	"testing"
+	"unicode/utf8"
+
+	"golang.org/x/text/transform"
+)
+
+type transformTest struct {
+	desc    string
+	szDst   int
+	atEOF   bool
+	repl    string
+	in      string
+	out     string // result string of first call to Transform
+	outFull string // transform of entire input string
+	err     error
+
+	t transform.Transformer
+}
+
+const large = 10240
+
+func (tt *transformTest) check(t *testing.T, i int) {
+	if tt.t == nil {
+		return
+	}
+	dst := make([]byte, tt.szDst)
+	src := []byte(tt.in)
+	nDst, nSrc, err := tt.t.Transform(dst, src, tt.atEOF)
+	if err != tt.err {
+		t.Errorf("%d:%s:error: got %v; want %v", i, tt.desc, err, tt.err)
+	}
+	if got := string(dst[:nDst]); got != tt.out {
+		t.Errorf("%d:%s:out: got %q; want %q", i, tt.desc, got, tt.out)
+	}
+
+	// Calls tt.t.Transform for the remainder of the input. We use this to test
+	// the nSrc return value.
+	out := make([]byte, large)
+	n := copy(out, dst[:nDst])
+	nDst, _, _ = tt.t.Transform(out[n:], src[nSrc:], true)
+	if got, want := string(out[:n+nDst]), tt.outFull; got != want {
+		t.Errorf("%d:%s:outFull: got %q; want %q", i, tt.desc, got, want)
+	}
+}
+
+func idem(r rune) rune { return r }
+
+func TestMap(t *testing.T) {
+	runes := []rune{'a', 'ç', '中', '\U00012345', 'a'}
+	// Default mapper used for this test.
+	rotate := Map(func(r rune) rune {
+		for i, m := range runes {
+			if m == r {
+				return runes[i+1]
+			}
+		}
+		return r
+	})
+
+	for i, tt := range []transformTest{{
+		desc:    "empty",
+		szDst:   large,
+		atEOF:   true,
+		in:      "",
+		out:     "",
+		outFull: "",
+		t:       rotate,
+	}, {
+		desc:    "no change",
+		szDst:   1,
+		atEOF:   true,
+		in:      "b",
+		out:     "b",
+		outFull: "b",
+		t:       rotate,
+	}, {
+		desc:    "short dst",
+		szDst:   2,
+		atEOF:   true,
+		in:      "aaaa",
+		out:     "ç",
+		outFull: "çççç",
+		err:     transform.ErrShortDst,
+		t:       rotate,
+	}, {
+		desc:    "short dst ascii, no change",
+		szDst:   2,
+		atEOF:   true,
+		in:      "bbb",
+		out:     "bb",
+		outFull: "bbb",
+		err:     transform.ErrShortDst,
+		t:       rotate,
+	}, {
+		desc:    "short dst writing error",
+		szDst:   2,
+		atEOF:   false,
+		in:      "a\x80",
+		out:     "ç",
+		outFull: "ç\ufffd",
+		err:     transform.ErrShortDst,
+		t:       rotate,
+	}, {
+		desc:    "short dst writing incomplete rune",
+		szDst:   2,
+		atEOF:   true,
+		in:      "a\xc0",
+		out:     "ç",
+		outFull: "ç\ufffd",
+		err:     transform.ErrShortDst,
+		t:       rotate,
+	}, {
+		desc:    "short dst, longer",
+		szDst:   5,
+		atEOF:   true,
+		in:      "Hellø",
+		out:     "Hell",
+		outFull: "Hellø",
+		err:     transform.ErrShortDst,
+		t:       rotate,
+	}, {
+		desc:    "short dst, single",
+		szDst:   1,
+		atEOF:   false,
+		in:      "ø",
+		out:     "",
+		outFull: "ø",
+		err:     transform.ErrShortDst,
+		t:       Map(idem),
+	}, {
+		desc:    "short dst, longer, writing error",
+		szDst:   8,
+		atEOF:   false,
+		in:      "\x80Hello\x80",
+		out:     "\ufffdHello",
+		outFull: "\ufffdHello\ufffd",
+		err:     transform.ErrShortDst,
+		t:       rotate,
+	}, {
+		desc:    "short src",
+		szDst:   2,
+		atEOF:   false,
+		in:      "a\xc0",
+		out:     "ç",
+		outFull: "ç\ufffd",
+		err:     transform.ErrShortSrc,
+		t:       rotate,
+	}, {
+		desc:    "invalid input, atEOF",
+		szDst:   large,
+		atEOF:   true,
+		in:      "\x80",
+		out:     "\ufffd",
+		outFull: "\ufffd",
+		t:       rotate,
+	}, {
+		desc:    "invalid input, !atEOF",
+		szDst:   large,
+		atEOF:   false,
+		in:      "\x80",
+		out:     "\ufffd",
+		outFull: "\ufffd",
+		t:       rotate,
+	}, {
+		desc:    "invalid input, incomplete rune atEOF",
+		szDst:   large,
+		atEOF:   true,
+		in:      "\xc0",
+		out:     "\ufffd",
+		outFull: "\ufffd",
+		t:       rotate,
+	}, {
+		desc:    "misc correct",
+		szDst:   large,
+		atEOF:   true,
+		in:      "a\U00012345 ç!",
+		out:     "ça 中!",
+		outFull: "ça 中!",
+		t:       rotate,
+	}, {
+		desc:    "misc correct and invalid",
+		szDst:   large,
+		atEOF:   true,
+		in:      "Hello\x80 w\x80orl\xc0d!\xc0",
+		out:     "Hello\ufffd w\ufffdorl\ufffdd!\ufffd",
+		outFull: "Hello\ufffd w\ufffdorl\ufffdd!\ufffd",
+		t:       rotate,
+	}, {
+		desc:    "misc correct and invalid, short src",
+		szDst:   large,
+		atEOF:   false,
+		in:      "Hello\x80 w\x80orl\xc0d!\xc0",
+		out:     "Hello\ufffd w\ufffdorl\ufffdd!",
+		outFull: "Hello\ufffd w\ufffdorl\ufffdd!\ufffd",
+		err:     transform.ErrShortSrc,
+		t:       rotate,
+	}, {
+		desc:    "misc correct and invalid, short src, replacing RuneError",
+		szDst:   large,
+		atEOF:   false,
+		in:      "Hel\ufffdlo\x80 w\x80orl\xc0d!\xc0",
+		out:     "Hel?lo? w?orl?d!",
+		outFull: "Hel?lo? w?orl?d!?",
+		err:     transform.ErrShortSrc,
+		t: Map(func(r rune) rune {
+			if r == utf8.RuneError {
+				return '?'
+			}
+			return r
+		}),
+	}} {
+		tt.check(t, i)
+	}
+}
+
+func TestRemove(t *testing.T) {
+	remove := Remove(Predicate(func(r rune) bool {
+		return strings.ContainsRune("aeiou\u0300\uFF24\U00012345", r)
+	}))
+
+	for i, tt := range []transformTest{
+		0: {
+			szDst:   large,
+			atEOF:   true,
+			in:      "",
+			out:     "",
+			outFull: "",
+			t:       remove,
+		},
+		1: {
+			szDst:   0,
+			atEOF:   true,
+			in:      "aaaa",
+			out:     "",
+			outFull: "",
+			t:       remove,
+		},
+		2: {
+			szDst:   1,
+			atEOF:   true,
+			in:      "aaaa",
+			out:     "",
+			outFull: "",
+			t:       remove,
+		},
+		3: {
+			szDst:   1,
+			atEOF:   true,
+			in:      "baaaa",
+			out:     "b",
+			outFull: "b",
+			t:       remove,
+		},
+		4: {
+			szDst:   2,
+			atEOF:   true,
+			in:      "açaaa",
+			out:     "ç",
+			outFull: "ç",
+			t:       remove,
+		},
+		5: {
+			szDst:   2,
+			atEOF:   true,
+			in:      "aaaç",
+			out:     "ç",
+			outFull: "ç",
+			t:       remove,
+		},
+		6: {
+			szDst:   2,
+			atEOF:   false,
+			in:      "a\x80",
+			out:     "",
+			outFull: "\ufffd",
+			err:     transform.ErrShortDst,
+			t:       remove,
+		},
+		7: {
+			szDst:   1,
+			atEOF:   true,
+			in:      "a\xc0",
+			out:     "",
+			outFull: "\ufffd",
+			err:     transform.ErrShortDst,
+			t:       remove,
+		},
+		8: {
+			szDst:   1,
+			atEOF:   false,
+			in:      "a\xc0",
+			out:     "",
+			outFull: "\ufffd",
+			err:     transform.ErrShortSrc,
+			t:       remove,
+		},
+		9: {
+			szDst:   large,
+			atEOF:   true,
+			in:      "\x80",
+			out:     "\ufffd",
+			outFull: "\ufffd",
+			t:       remove,
+		},
+		10: {
+			szDst:   large,
+			atEOF:   false,
+			in:      "\x80",
+			out:     "\ufffd",
+			outFull: "\ufffd",
+			t:       remove,
+		},
+		11: {
+			szDst:   large,
+			atEOF:   true,
+			in:      "\xc0",
+			out:     "\ufffd",
+			outFull: "\ufffd",
+			t:       remove,
+		},
+		12: {
+			szDst:   large,
+			atEOF:   true,
+			in:      "Hello \U00012345world!",
+			out:     "Hll wrld!",
+			outFull: "Hll wrld!",
+			t:       remove,
+		},
+		13: {
+			szDst:   large,
+			atEOF:   true,
+			in:      "Hello\x80 w\x80orl\xc0d!\xc0",
+			out:     "Hll\ufffd w\ufffdrl\ufffdd!\ufffd",
+			outFull: "Hll\ufffd w\ufffdrl\ufffdd!\ufffd",
+			t:       remove,
+		},
+		14: {
+			szDst:   large,
+			atEOF:   false,
+			in:      "Hello\x80 w\x80orl\xc0d!\xc0",
+			out:     "Hll\ufffd w\ufffdrl\ufffdd!",
+			outFull: "Hll\ufffd w\ufffdrl\ufffdd!\ufffd",
+			err:     transform.ErrShortSrc,
+			t:       remove,
+		},
+		15: {
+			szDst:   large,
+			atEOF:   false,
+			in:      "Hel\ufffdlo\x80 w\x80orl\xc0d!\xc0",
+			out:     "Hello world!",
+			outFull: "Hello world!",
+			err:     transform.ErrShortSrc,
+			t:       Remove(Predicate(func(r rune) bool { return r == utf8.RuneError })),
+		},
+		16: {
+			szDst:   4,
+			atEOF:   true,
+			in:      "Hellø",
+			out:     "Hll",
+			outFull: "Hllø",
+			err:     transform.ErrShortDst,
+			t:       remove,
+		},
+		17: {
+			szDst:   4,
+			atEOF:   false,
+			in:      "Hellø",
+			out:     "Hll",
+			outFull: "Hllø",
+			err:     transform.ErrShortDst,
+			t:       remove,
+		},
+		18: {
+			szDst:   8,
+			atEOF:   false,
+			in:      "\x80Hello\uFF24\x80",
+			out:     "\ufffdHll",
+			outFull: "\ufffdHll\ufffd",
+			err:     transform.ErrShortDst,
+			t:       remove,
+		},
+	} {
+		tt.check(t, i)
+	}
+}
+
+func TestMapAlloc(t *testing.T) {
+	if n := testing.AllocsPerRun(3, func() {
+		Map(idem).Transform(nil, nil, false)
+	}); n > 0 {
+		t.Errorf("got %f; want 0", n)
+	}
+}
+
+func rmNop(r rune) bool { return false }
+
+func TestRemoveAlloc(t *testing.T) {
+	if n := testing.AllocsPerRun(3, func() {
+		Remove(Predicate(rmNop)).Transform(nil, nil, false)
+	}); n > 0 {
+		t.Errorf("got %f; want 0", n)
+	}
+}
+
+func BenchmarkRemove(b *testing.B) {
+	dst := make([]byte, len(input))
+	src := []byte(input)
+
+	r := Remove(Predicate(func(r rune) bool { return r == 'e' }))
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		r.Transform(dst, src, true)
+	}
+}
+
+func BenchmarkMapAll(b *testing.B) {
+	dst := make([]byte, 2*len(input))
+	src := []byte(input)
+
+	r := Map(func(r rune) rune { return 'a' })
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		r.Transform(dst, src, true)
+	}
+}
+
+func BenchmarkMapNone(b *testing.B) {
+	dst := make([]byte, 2*len(input))
+	src := []byte(input)
+
+	r := Map(func(r rune) rune { return r })
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		r.Transform(dst, src, true)
+	}
+}
+
+var (
+	input = strings.Repeat("Thé qüick brøwn føx jumps øver the lazy døg. ", 100)
+)