diff --git a/internal/edit/edit.go b/internal/edit/edit.go
new file mode 100644
index 0000000..effb326
--- /dev/null
+++ b/internal/edit/edit.go
@@ -0,0 +1,96 @@
+// Copyright 2017 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 edit implements buffered position-based editing of byte slices.
+package edit
+
+import (
+	"fmt"
+	"sort"
+)
+
+// A Buffer is a queue of edits to apply to a given byte slice.
+type Buffer struct {
+	old []byte
+	q   edits
+}
+
+// An edit records a single text modification: change the bytes in [start,end) to new.
+type edit struct {
+	start int
+	end   int
+	new   string
+}
+
+// An edits is a list of edits that is sortable by start offset, breaking ties by end offset.
+type edits []edit
+
+func (x edits) Len() int      { return len(x) }
+func (x edits) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
+func (x edits) Less(i, j int) bool {
+	if x[i].start != x[j].start {
+		return x[i].start < x[j].start
+	}
+	return x[i].end < x[j].end
+}
+
+// NewBuffer returns a new buffer to accumulate changes to an initial data slice.
+// The returned buffer maintains a reference to the data, so the caller must ensure
+// the data is not modified until after the Buffer is done being used.
+func NewBuffer(old []byte) *Buffer {
+	return &Buffer{old: old}
+}
+
+// Insert inserts the new string at old[pos:pos].
+func (b *Buffer) Insert(pos int, new string) {
+	if pos < 0 || pos > len(b.old) {
+		panic("invalid edit position")
+	}
+	b.q = append(b.q, edit{pos, pos, new})
+}
+
+// Delete deletes the text old[start:end].
+func (b *Buffer) Delete(start, end int) {
+	if end < start || start < 0 || end > len(b.old) {
+		panic("invalid edit position")
+	}
+	b.q = append(b.q, edit{start, end, ""})
+}
+
+// Replace replaces old[start:end] with new.
+func (b *Buffer) Replace(start, end int, new string) {
+	if end < start || start < 0 || end > len(b.old) {
+		panic("invalid edit position")
+	}
+	b.q = append(b.q, edit{start, end, new})
+}
+
+// Bytes returns a new byte slice containing the original data
+// with the queued edits applied.
+func (b *Buffer) Bytes() []byte {
+	// Sort edits by starting position and then by ending position.
+	// Breaking ties by ending position allows insertions at point x
+	// to be applied before a replacement of the text at [x, y).
+	sort.Stable(b.q)
+
+	var new []byte
+	offset := 0
+	for i, e := range b.q {
+		if e.start < offset {
+			e0 := b.q[i-1]
+			panic(fmt.Sprintf("overlapping edits: [%d,%d)->%q, [%d,%d)->%q", e0.start, e0.end, e0.new, e.start, e.end, e.new))
+		}
+		new = append(new, b.old[offset:e.start]...)
+		offset = e.end
+		new = append(new, e.new...)
+	}
+	new = append(new, b.old[offset:]...)
+	return new
+}
+
+// String returns a string containing the original data
+// with the queued edits applied.
+func (b *Buffer) String() string {
+	return string(b.Bytes())
+}
diff --git a/internal/edit/edit_test.go b/internal/edit/edit_test.go
new file mode 100644
index 0000000..0e0c564
--- /dev/null
+++ b/internal/edit/edit_test.go
@@ -0,0 +1,28 @@
+// Copyright 2017 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 edit
+
+import "testing"
+
+func TestEdit(t *testing.T) {
+	b := NewBuffer([]byte("0123456789"))
+	b.Insert(8, ",7½,")
+	b.Replace(9, 10, "the-end")
+	b.Insert(10, "!")
+	b.Insert(4, "3.14,")
+	b.Insert(4, "π,")
+	b.Insert(4, "3.15,")
+	b.Replace(3, 4, "three,")
+	want := "012three,3.14,π,3.15,4567,7½,8the-end!"
+
+	s := b.String()
+	if s != want {
+		t.Errorf("b.String() = %q, want %q", s, want)
+	}
+	sb := b.Bytes()
+	if string(sb) != want {
+		t.Errorf("b.Bytes() = %q, want %q", sb, want)
+	}
+}