diff --git a/Blueprints b/Blueprints
index c892b46..93357fa 100644
--- a/Blueprints
+++ b/Blueprints
@@ -43,6 +43,7 @@
         "parser/sort.go",
     ],
     testSrcs: [
+        "parser/modify_test.go",
         "parser/parser_test.go",
         "parser/printer_test.go",
     ],
diff --git a/parser/ast.go b/parser/ast.go
index 7f94efb..b5053bb 100644
--- a/parser/ast.go
+++ b/parser/ast.go
@@ -21,9 +21,9 @@
 )
 
 type Node interface {
-	// Pos returns the position of the first token in the Expression
+	// Pos returns the position of the first token in the Node
 	Pos() scanner.Position
-	// End returns the position of the beginning of the last token in the Expression
+	// End returns the position of the character after the last token in the Node
 	End() scanner.Position
 }
 
@@ -220,7 +220,7 @@
 }
 
 func (x *Variable) Pos() scanner.Position { return x.NamePos }
-func (x *Variable) End() scanner.Position { return x.NamePos }
+func (x *Variable) End() scanner.Position { return endPos(x.NamePos, len(x.Name)) }
 
 func (x *Variable) Copy() Expression {
 	ret := *x
@@ -244,7 +244,7 @@
 }
 
 func (x *Map) Pos() scanner.Position { return x.LBracePos }
-func (x *Map) End() scanner.Position { return x.RBracePos }
+func (x *Map) End() scanner.Position { return endPos(x.RBracePos, 1) }
 
 func (x *Map) Copy() Expression {
 	ret := *x
@@ -302,7 +302,7 @@
 }
 
 func (x *List) Pos() scanner.Position { return x.LBracePos }
-func (x *List) End() scanner.Position { return x.RBracePos }
+func (x *List) End() scanner.Position { return endPos(x.RBracePos, 1) }
 
 func (x *List) Copy() Expression {
 	ret := *x
@@ -334,7 +334,7 @@
 }
 
 func (x *String) Pos() scanner.Position { return x.LiteralPos }
-func (x *String) End() scanner.Position { return x.LiteralPos }
+func (x *String) End() scanner.Position { return endPos(x.LiteralPos, len(x.Value)+2) }
 
 func (x *String) Copy() Expression {
 	ret := *x
@@ -356,10 +356,11 @@
 type Int64 struct {
 	LiteralPos scanner.Position
 	Value      int64
+	Token      string
 }
 
 func (x *Int64) Pos() scanner.Position { return x.LiteralPos }
-func (x *Int64) End() scanner.Position { return x.LiteralPos }
+func (x *Int64) End() scanner.Position { return endPos(x.LiteralPos, len(x.Token)) }
 
 func (x *Int64) Copy() Expression {
 	ret := *x
@@ -381,10 +382,11 @@
 type Bool struct {
 	LiteralPos scanner.Position
 	Value      bool
+	Token      string
 }
 
 func (x *Bool) Pos() scanner.Position { return x.LiteralPos }
-func (x *Bool) End() scanner.Position { return x.LiteralPos }
+func (x *Bool) End() scanner.Position { return endPos(x.LiteralPos, len(x.Token)) }
 
 func (x *Bool) Copy() Expression {
 	ret := *x
@@ -422,7 +424,8 @@
 func (c Comment) End() scanner.Position {
 	pos := c.Slash
 	for _, comment := range c.Comment {
-		pos.Offset += len(comment)
+		pos.Offset += len(comment) + 1
+		pos.Column = len(comment) + 1
 	}
 	pos.Line += len(c.Comment) - 1
 	return pos
@@ -472,3 +475,9 @@
 
 	return string(buf)
 }
+
+func endPos(pos scanner.Position, n int) scanner.Position {
+	pos.Offset += n
+	pos.Column += n
+	return pos
+}
diff --git a/parser/modify.go b/parser/modify.go
index 08a3f3f..3051f66 100644
--- a/parser/modify.go
+++ b/parser/modify.go
@@ -14,7 +14,12 @@
 
 package parser
 
-import "fmt"
+import (
+	"fmt"
+	"io"
+	"math"
+	"sort"
+)
 
 func AddStringToList(list *List, s string) (modified bool) {
 	for _, v := range list.Values {
@@ -50,3 +55,66 @@
 
 	return false
 }
+
+// A Patch represents a region of a text buffer to be replaced [Start, End) and its Replacement
+type Patch struct {
+	Start, End  int
+	Replacement string
+}
+
+// A PatchList is a list of sorted, non-overlapping Patch objects
+type PatchList []Patch
+
+type PatchOverlapError error
+
+// Add adds a Patch to a PatchList.  It returns a PatchOverlapError if the patch cannot be added.
+func (list *PatchList) Add(start, end int, replacement string) error {
+	patch := Patch{start, end, replacement}
+	if patch.Start > patch.End {
+		return fmt.Errorf("invalid patch, start %d is after end %d", patch.Start, patch.End)
+	}
+	for _, p := range *list {
+		if (patch.Start >= p.Start && patch.Start < p.End) ||
+			(patch.End >= p.Start && patch.End < p.End) ||
+			(p.Start >= patch.Start && p.Start < patch.End) ||
+			(p.Start == patch.Start && p.End == patch.End) {
+			return PatchOverlapError(fmt.Errorf("new patch %d-%d overlaps with existing patch %d-%d",
+				patch.Start, patch.End, p.Start, p.End))
+		}
+	}
+	*list = append(*list, patch)
+	list.sort()
+	return nil
+}
+
+func (list *PatchList) sort() {
+	sort.SliceStable(*list,
+		func(i, j int) bool {
+			return (*list)[i].Start < (*list)[j].Start
+		})
+}
+
+// Apply applies all the Patch objects in PatchList to the data from an input ReaderAt to an output Writer.
+func (list *PatchList) Apply(in io.ReaderAt, out io.Writer) error {
+	var offset int64
+	for _, patch := range *list {
+		toWrite := int64(patch.Start) - offset
+		written, err := io.Copy(out, io.NewSectionReader(in, offset, toWrite))
+		if err != nil {
+			return err
+		}
+		offset += toWrite
+		if written != toWrite {
+			return fmt.Errorf("unexpected EOF at %d", offset)
+		}
+
+		_, err = io.WriteString(out, patch.Replacement)
+		if err != nil {
+			return err
+		}
+
+		offset += int64(patch.End - patch.Start)
+	}
+	_, err := io.Copy(out, io.NewSectionReader(in, offset, math.MaxInt64-offset))
+	return err
+}
diff --git a/parser/modify_test.go b/parser/modify_test.go
new file mode 100644
index 0000000..95fc293
--- /dev/null
+++ b/parser/modify_test.go
@@ -0,0 +1,65 @@
+// Copyright 2018 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package parser
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestPatchList(t *testing.T) {
+	expectOverlap := func(err error) {
+		t.Helper()
+		if _, ok := err.(PatchOverlapError); !ok {
+			t.Error("missing PatchOverlapError")
+		}
+	}
+
+	expectOk := func(err error) {
+		t.Helper()
+		if err != nil {
+			t.Error(err)
+		}
+	}
+
+	in := []byte("abcdefghijklmnopqrstuvwxyz")
+
+	patchlist := PatchList{}
+	expectOk(patchlist.Add(0, 3, "ABC"))
+	expectOk(patchlist.Add(12, 15, "MNO"))
+	expectOk(patchlist.Add(24, 26, "Z"))
+	expectOk(patchlist.Add(15, 15, "_"))
+
+	expectOverlap(patchlist.Add(0, 3, "x"))
+	expectOverlap(patchlist.Add(12, 13, "x"))
+	expectOverlap(patchlist.Add(13, 14, "x"))
+	expectOverlap(patchlist.Add(14, 15, "x"))
+	expectOverlap(patchlist.Add(11, 13, "x"))
+	expectOverlap(patchlist.Add(12, 15, "x"))
+	expectOverlap(patchlist.Add(11, 15, "x"))
+	expectOverlap(patchlist.Add(15, 15, "x"))
+
+	if t.Failed() {
+		return
+	}
+
+	buf := new(bytes.Buffer)
+	patchlist.Apply(bytes.NewReader(in), buf)
+	expected := "ABCdefghijklMNO_pqrstuvwxZ"
+	got := buf.String()
+	if got != expected {
+		t.Errorf("expected %q, got %q", expected, got)
+	}
+}
diff --git a/parser/parser.go b/parser/parser.go
index 728afbe..e832e1a 100644
--- a/parser/parser.go
+++ b/parser/parser.go
@@ -360,6 +360,7 @@
 				v.Value += e2.(*String).Value
 			case *Int64:
 				v.Value += e2.(*Int64).Value
+				v.Token = ""
 			case *List:
 				v.Values = append(v.Values, e2.(*List).Values...)
 			case *Map:
@@ -469,6 +470,7 @@
 		value = &Bool{
 			LiteralPos: p.scanner.Position,
 			Value:      text == "true",
+			Token:      text,
 		}
 	default:
 		if p.eval {
@@ -528,6 +530,7 @@
 	value := &Int64{
 		LiteralPos: literalPos,
 		Value:      i,
+		Token:      str,
 	}
 	p.accept(scanner.Int)
 	return value
diff --git a/parser/parser_test.go b/parser/parser_test.go
index d740184..6377dc1 100644
--- a/parser/parser_test.go
+++ b/parser/parser_test.go
@@ -17,6 +17,8 @@
 import (
 	"bytes"
 	"reflect"
+	"strconv"
+	"strings"
 	"testing"
 	"text/scanner"
 )
@@ -99,6 +101,7 @@
 							Value: &Bool{
 								LiteralPos: mkpos(20, 3, 12),
 								Value:      true,
+								Token:      "true",
 							},
 						},
 					},
@@ -128,6 +131,7 @@
 							Value: &Int64{
 								LiteralPos: mkpos(17, 3, 9),
 								Value:      4,
+								Token:      "4",
 							},
 						},
 					},
@@ -221,6 +225,7 @@
 										Value: &Bool{
 											LiteralPos: mkpos(33, 4, 13),
 											Value:      true,
+											Token:      "true",
 										},
 									},
 									{
@@ -239,6 +244,7 @@
 										Value: &Int64{
 											LiteralPos: mkpos(65, 6, 10),
 											Value:      36,
+											Token:      "36",
 										},
 									},
 								},
@@ -273,6 +279,7 @@
 							Value: &Bool{
 								LiteralPos: mkpos(60, 5, 12),
 								Value:      true,
+								Token:      "true",
 							},
 						},
 					},
@@ -350,6 +357,7 @@
 							Value: &Int64{
 								LiteralPos: mkpos(33, 4, 9),
 								Value:      4,
+								Token:      "4",
 							},
 						},
 					},
@@ -378,6 +386,7 @@
 							Value: &Int64{
 								LiteralPos: mkpos(73, 9, 9),
 								Value:      -5,
+								Token:      "-5",
 							},
 						},
 					},
@@ -637,6 +646,7 @@
 						&Int64{
 							LiteralPos: mkpos(9, 2, 9),
 							Value:      -4,
+							Token:      "-4",
 						},
 						&Operator{
 							OperatorPos: mkpos(17, 2, 17),
@@ -649,10 +659,12 @@
 								&Int64{
 									LiteralPos: mkpos(14, 2, 14),
 									Value:      -5,
+									Token:      "-5",
 								},
 								&Int64{
 									LiteralPos: mkpos(19, 2, 19),
 									Value:      6,
+									Token:      "6",
 								},
 							},
 						},
@@ -669,6 +681,7 @@
 						&Int64{
 							LiteralPos: mkpos(9, 2, 9),
 							Value:      -4,
+							Token:      "-4",
 						},
 						&Operator{
 							OperatorPos: mkpos(17, 2, 17),
@@ -681,10 +694,12 @@
 								&Int64{
 									LiteralPos: mkpos(14, 2, 14),
 									Value:      -5,
+									Token:      "-5",
 								},
 								&Int64{
 									LiteralPos: mkpos(19, 2, 19),
 									Value:      6,
+									Token:      "6",
 								},
 							},
 						},
@@ -712,10 +727,12 @@
 				Value: &Int64{
 					LiteralPos: mkpos(9, 2, 9),
 					Value:      1000000,
+					Token:      "1000000",
 				},
 				OrigValue: &Int64{
 					LiteralPos: mkpos(9, 2, 9),
 					Value:      1000000,
+					Token:      "1000000",
 				},
 				Assigner:   "=",
 				Referenced: true,
@@ -730,6 +747,7 @@
 					Value: &Int64{
 						LiteralPos: mkpos(9, 2, 9),
 						Value:      1000000,
+						Token:      "1000000",
 					},
 				},
 				OrigValue: &Variable{
@@ -738,6 +756,7 @@
 					Value: &Int64{
 						LiteralPos: mkpos(9, 2, 9),
 						Value:      1000000,
+						Token:      "1000000",
 					},
 				},
 				Assigner:   "=",
@@ -761,6 +780,7 @@
 							Value: &Int64{
 								LiteralPos: mkpos(9, 2, 9),
 								Value:      1000000,
+								Token:      "1000000",
 							},
 						},
 						&Variable{
@@ -772,6 +792,7 @@
 								Value: &Int64{
 									LiteralPos: mkpos(9, 2, 9),
 									Value:      1000000,
+									Token:      "1000000",
 								},
 							},
 						},
@@ -791,6 +812,7 @@
 							Value: &Int64{
 								LiteralPos: mkpos(9, 2, 9),
 								Value:      1000000,
+								Token:      "1000000",
 							},
 						},
 						&Variable{
@@ -802,6 +824,7 @@
 								Value: &Int64{
 									LiteralPos: mkpos(9, 2, 9),
 									Value:      1000000,
+									Token:      "1000000",
 								},
 							},
 						},
@@ -833,6 +856,7 @@
 										Value: &Int64{
 											LiteralPos: mkpos(9, 2, 9),
 											Value:      1000000,
+											Token:      "1000000",
 										},
 									},
 									&Variable{
@@ -844,6 +868,7 @@
 											Value: &Int64{
 												LiteralPos: mkpos(9, 2, 9),
 												Value:      1000000,
+												Token:      "1000000",
 											},
 										},
 									},
@@ -856,6 +881,7 @@
 							Value: &Int64{
 								LiteralPos: mkpos(9, 2, 9),
 								Value:      1000000,
+								Token:      "1000000",
 							},
 						},
 					},
@@ -883,6 +909,7 @@
 								Value: &Int64{
 									LiteralPos: mkpos(9, 2, 9),
 									Value:      1000000,
+									Token:      "1000000",
 								},
 							},
 							&Variable{
@@ -894,6 +921,7 @@
 									Value: &Int64{
 										LiteralPos: mkpos(9, 2, 9),
 										Value:      1000000,
+										Token:      "1000000",
 									},
 								},
 							},
@@ -912,6 +940,7 @@
 					Value: &Int64{
 						LiteralPos: mkpos(9, 2, 9),
 						Value:      1000000,
+						Token:      "1000000",
 					},
 				},
 				OrigValue: &Variable{
@@ -920,6 +949,7 @@
 					Value: &Int64{
 						LiteralPos: mkpos(9, 2, 9),
 						Value:      1000000,
+						Token:      "1000000",
 					},
 				},
 				Assigner: "+=",
@@ -985,48 +1015,110 @@
 }
 
 func TestParseValidInput(t *testing.T) {
-	for _, testCase := range validParseTestCases {
-		r := bytes.NewBufferString(testCase.input)
-		file, errs := ParseAndEval("", r, NewScope(nil))
-		if len(errs) != 0 {
-			t.Errorf("test case: %s", testCase.input)
-			t.Errorf("unexpected errors:")
-			for _, err := range errs {
-				t.Errorf("  %s", err)
-			}
-			t.FailNow()
-		}
-
-		if len(file.Defs) == len(testCase.defs) {
-			for i := range file.Defs {
-				if !reflect.DeepEqual(file.Defs[i], testCase.defs[i]) {
-					t.Errorf("test case: %s", testCase.input)
-					t.Errorf("incorrect defintion %d:", i)
-					t.Errorf("  expected: %s", testCase.defs[i])
-					t.Errorf("       got: %s", file.Defs[i])
+	for i, testCase := range validParseTestCases {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			r := bytes.NewBufferString(testCase.input)
+			file, errs := ParseAndEval("", r, NewScope(nil))
+			if len(errs) != 0 {
+				t.Errorf("test case: %s", testCase.input)
+				t.Errorf("unexpected errors:")
+				for _, err := range errs {
+					t.Errorf("  %s", err)
 				}
+				t.FailNow()
 			}
-		} else {
-			t.Errorf("test case: %s", testCase.input)
-			t.Errorf("length mismatch, expected %d definitions, got %d",
-				len(testCase.defs), len(file.Defs))
-		}
 
-		if len(file.Comments) == len(testCase.comments) {
-			for i := range file.Comments {
-				if !reflect.DeepEqual(file.Comments[i], testCase.comments[i]) {
-					t.Errorf("test case: %s", testCase.input)
-					t.Errorf("incorrect comment %d:", i)
-					t.Errorf("  expected: %s", testCase.comments[i])
-					t.Errorf("       got: %s", file.Comments[i])
+			if len(file.Defs) == len(testCase.defs) {
+				for i := range file.Defs {
+					if !reflect.DeepEqual(file.Defs[i], testCase.defs[i]) {
+						t.Errorf("test case: %s", testCase.input)
+						t.Errorf("incorrect defintion %d:", i)
+						t.Errorf("  expected: %s", testCase.defs[i])
+						t.Errorf("       got: %s", file.Defs[i])
+					}
 				}
+			} else {
+				t.Errorf("test case: %s", testCase.input)
+				t.Errorf("length mismatch, expected %d definitions, got %d",
+					len(testCase.defs), len(file.Defs))
 			}
-		} else {
-			t.Errorf("test case: %s", testCase.input)
-			t.Errorf("length mismatch, expected %d comments, got %d",
-				len(testCase.comments), len(file.Comments))
-		}
+
+			if len(file.Comments) == len(testCase.comments) {
+				for i := range file.Comments {
+					if !reflect.DeepEqual(file.Comments[i], testCase.comments[i]) {
+						t.Errorf("test case: %s", testCase.input)
+						t.Errorf("incorrect comment %d:", i)
+						t.Errorf("  expected: %s", testCase.comments[i])
+						t.Errorf("       got: %s", file.Comments[i])
+					}
+				}
+			} else {
+				t.Errorf("test case: %s", testCase.input)
+				t.Errorf("length mismatch, expected %d comments, got %d",
+					len(testCase.comments), len(file.Comments))
+			}
+		})
 	}
 }
 
 // TODO: Test error strings
+
+func TestParserEndPos(t *testing.T) {
+	in := `
+		module {
+			string: "string",
+			stringexp: "string1" + "string2",
+			int: -1,
+			intexp: -1 + 2,
+			list: ["a", "b"],
+			listexp: ["c"] + ["d"],
+			multilinelist: [
+				"e",
+				"f",
+			],
+			map: {
+				prop: "abc",
+			},
+		}
+	`
+
+	// Strip each line to make it easier to compute the previous "," from each property
+	lines := strings.Split(in, "\n")
+	for i := range lines {
+		lines[i] = strings.TrimSpace(lines[i])
+	}
+	in = strings.Join(lines, "\n")
+
+	r := bytes.NewBufferString(in)
+
+	file, errs := ParseAndEval("", r, NewScope(nil))
+	if len(errs) != 0 {
+		t.Errorf("unexpected errors:")
+		for _, err := range errs {
+			t.Errorf("  %s", err)
+		}
+		t.FailNow()
+	}
+
+	mod := file.Defs[0].(*Module)
+	modEnd := mkpos(len(in)-1, len(lines)-1, 2)
+	if mod.End() != modEnd {
+		t.Errorf("expected mod.End() %s, got %s", modEnd, mod.End())
+	}
+
+	nextPos := make([]scanner.Position, len(mod.Properties))
+	for i := 0; i < len(mod.Properties)-1; i++ {
+		nextPos[i] = mod.Properties[i+1].Pos()
+	}
+	nextPos[len(mod.Properties)-1] = mod.RBracePos
+
+	for i, cur := range mod.Properties {
+		endOffset := nextPos[i].Offset - len(",\n")
+		endLine := nextPos[i].Line - 1
+		endColumn := len(lines[endLine-1]) // scanner.Position.Line is starts at 1
+		endPos := mkpos(endOffset, endLine, endColumn)
+		if cur.End() != endPos {
+			t.Errorf("expected property %s End() %s@%d, got %s@%d", cur.Name, endPos, endPos.Offset, cur.End(), cur.End().Offset)
+		}
+	}
+}
diff --git a/proptools/proptools.go b/proptools/proptools.go
index e071b3c..f4da29e 100644
--- a/proptools/proptools.go
+++ b/proptools/proptools.go
@@ -66,20 +66,32 @@
 	return &s
 }
 
-// Bool takes a pointer to a bool and returns true iff the pointer is non-nil and points to a true
-// value.
-func Bool(b *bool) bool {
+// BoolDefault takes a pointer to a bool and returns the value pointed to by the pointer if it is non-nil,
+// or def if the pointer is nil.
+func BoolDefault(b *bool, def bool) bool {
 	if b != nil {
 		return *b
 	}
-	return false
+	return def
+}
+
+// Bool takes a pointer to a bool and returns true iff the pointer is non-nil and points to a true
+// value.
+func Bool(b *bool) bool {
+	return BoolDefault(b, false)
+}
+
+// String takes a pointer to a string and returns the value of the string if the pointer is non-nil,
+// or def if the pointer is nil.
+func StringDefault(s *string, def string) string {
+	if s != nil {
+		return *s
+	}
+	return def
 }
 
 // String takes a pointer to a string and returns the value of the string if the pointer is non-nil,
 // or an empty string.
 func String(s *string) string {
-	if s != nil {
-		return *s
-	}
-	return ""
+	return StringDefault(s, "")
 }