diff --git a/cue/token/position.go b/cue/token/position.go
new file mode 100644
index 0000000..f5ec34f
--- /dev/null
+++ b/cue/token/position.go
@@ -0,0 +1,564 @@
+// Copyright 2018 The CUE Authors
+//
+// 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 token
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+// -----------------------------------------------------------------------------
+// Positions
+
+// Position describes an arbitrary source position
+// including the file, line, and column location.
+// A Position is valid if the line number is > 0.
+type Position struct {
+	Filename string // filename, if any
+	Offset   int    // offset, starting at 0
+	Line     int    // line number, starting at 1
+	Column   int    // column number, starting at 1 (byte count)
+	// RelPos   Pos // relative position information
+}
+
+// IsValid reports whether the position is valid.
+func (pos *Position) IsValid() bool { return pos.Line > 0 }
+
+// String returns a string in one of several forms:
+//
+//	file:line:column    valid position with file name
+//	line:column         valid position without file name
+//	file                invalid position with file name
+//	-                   invalid position without file name
+//
+func (pos Position) String() string {
+	s := pos.Filename
+	if pos.IsValid() {
+		if s != "" {
+			s += ":"
+		}
+		s += fmt.Sprintf("%d:%d", pos.Line, pos.Column)
+	}
+	if s == "" {
+		s = "-"
+	}
+	return s
+}
+
+// Pos is a compact encoding of a source position within a file set, as well as
+// relative positioning information. It can be converted into a Position for a
+// more convenient, but much larger, representation.
+//
+// The Pos value for a given file is a number in the range [base, base+size],
+// where base and size are specified when adding the file to the file set via
+// AddFile.
+//
+// To create the Pos value for a specific source offset (measured in bytes),
+// first add the respective file to the current file set using FileSet.AddFile
+// and then call File.Pos(offset) for that file. Given a Pos value p for a
+// specific file set fset, the corresponding Position value is obtained by
+// calling fset.Position(p).
+//
+// Pos values can be compared directly with the usual comparison operators: If
+// two Pos values p and q are in the same file, comparing p and q is equivalent
+// to comparing the respective source file offsets. If p and q are in different
+// files, p < q is true if the file implied by p was added to the respective
+// file set before the file implied by cue.
+type Pos int
+
+// NoPos is the zero value for Pos; there is no file and line information
+// associated with it, and NoPos().IsValid() is false. NoPos is always
+// smaller than any other Pos value. The corresponding Position value
+// for NoPos is the zero value for Position.
+const NoPos Pos = 0
+
+// RelPos indicates the relative position of token to the previous token.
+type RelPos int
+
+const (
+	// NoRelPos indicates no relative position is specified.
+	NoRelPos RelPos = iota
+
+	// Elided indicates that the token for which this position is defined is
+	// not rendered at all.
+	Elided
+
+	// NoSpace indicates there is no whitespace after this token.
+	NoSpace
+
+	// Blank means there is horizontal space after this token.
+	Blank
+
+	// Newline means there is a single newline after this token.
+	Newline
+
+	// NewSection means there are two or more newlines after this token.
+	NewSection
+
+	relMask  Pos = 0xf
+	relShift     = 4
+)
+
+var relNames = []string{
+	"invalid", "elided", "nospace", "blank", "newline", "section",
+}
+
+func (p RelPos) String() string { return relNames[p] }
+
+// HasRelPos repors whether p has a relative position.
+func (p Pos) HasRelPos() bool {
+	return p&relMask != 0
+
+}
+
+func (p Pos) Add(n int) Pos {
+	return p + toPos(index(n))
+}
+
+// IsValid reports whether the position is valid.
+func (p Pos) IsValid() bool {
+	return p != NoPos
+}
+
+// IsNewline reports whether the relative information suggests this node should
+// be printed on a new lien.
+func (p Pos) IsNewline() bool {
+	return p.RelPos() >= Newline
+}
+
+func (p Pos) WithRel(rel RelPos) Pos {
+	return p&^relMask | Pos(rel)
+}
+
+func (p Pos) RelPos() RelPos {
+	return RelPos(p & relMask)
+}
+
+func (p Pos) index() index {
+	return index(p) >> relShift
+}
+
+func toPos(x index) Pos {
+	return (Pos(x) << relShift)
+}
+
+// -----------------------------------------------------------------------------
+// File
+
+type index int
+
+// A File is a handle for a file belonging to a FileSet.
+// A File has a name, size, and line offset table.
+type File struct {
+	set  *FileSet
+	name string // file name as provided to AddFile
+	base index  // Pos index range for this file is [base...base+size]
+	size index  // file size as provided to AddFile
+
+	// lines and infos are protected by set.mutex
+	lines []index // lines contains the offset of the first character for each line (the first entry is always 0)
+	infos []lineInfo
+}
+
+// Name returns the file name of file f as registered with AddFile.
+func (f *File) Name() string {
+	return f.name
+}
+
+// Base returns the base offset of file f as registered with AddFile.
+func (f *File) Base() int {
+	return int(f.base)
+}
+
+// Size returns the size of file f as registered with AddFile.
+func (f *File) Size() int {
+	return int(f.size)
+}
+
+// LineCount returns the number of lines in file f.
+func (f *File) LineCount() int {
+	f.set.mutex.RLock()
+	n := len(f.lines)
+	f.set.mutex.RUnlock()
+	return n
+}
+
+// AddLine adds the line offset for a new line.
+// The line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise the line offset is ignored.
+//
+func (f *File) AddLine(offset int) {
+	x := index(offset)
+	f.set.mutex.Lock()
+	if i := len(f.lines); (i == 0 || f.lines[i-1] < x) && x < f.size {
+		f.lines = append(f.lines, x)
+	}
+	f.set.mutex.Unlock()
+}
+
+// MergeLine merges a line with the following line. It is akin to replacing
+// the newline character at the end of the line with a space (to not change the
+// remaining offsets). To obtain the line number, consult e.g. Position.Line.
+// MergeLine will panic if given an invalid line number.
+//
+func (f *File) MergeLine(line int) {
+	if line <= 0 {
+		panic("illegal line number (line numbering starts at 1)")
+	}
+	f.set.mutex.Lock()
+	defer f.set.mutex.Unlock()
+	if line >= len(f.lines) {
+		panic("illegal line number")
+	}
+	// To merge the line numbered <line> with the line numbered <line+1>,
+	// we need to remove the entry in lines corresponding to the line
+	// numbered <line+1>. The entry in lines corresponding to the line
+	// numbered <line+1> is located at index <line>, since indices in lines
+	// are 0-based and line numbers are 1-based.
+	copy(f.lines[line:], f.lines[line+1:])
+	f.lines = f.lines[:len(f.lines)-1]
+}
+
+// SetLines sets the line offsets for a file and reports whether it succeeded.
+// The line offsets are the offsets of the first character of each line;
+// for instance for the content "ab\nc\n" the line offsets are {0, 3}.
+// An empty file has an empty line offset table.
+// Each line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise SetLines fails and returns
+// false.
+// Callers must not mutate the provided slice after SetLines returns.
+//
+func (f *File) SetLines(lines []int) bool {
+	// verify validity of lines table
+	size := f.size
+	for i, offset := range lines {
+		if i > 0 && offset <= lines[i-1] || size <= index(offset) {
+			return false
+		}
+	}
+
+	// set lines table
+	f.set.mutex.Lock()
+	f.lines = f.lines[:0]
+	for _, l := range lines {
+		f.lines = append(f.lines, index(l))
+	}
+	f.set.mutex.Unlock()
+	return true
+}
+
+// SetLinesForContent sets the line offsets for the given file content.
+// It ignores position-altering //line comments.
+func (f *File) SetLinesForContent(content []byte) {
+	var lines []index
+	line := index(0)
+	for offset, b := range content {
+		if line >= 0 {
+			lines = append(lines, line)
+		}
+		line = -1
+		if b == '\n' {
+			line = index(offset) + 1
+		}
+	}
+
+	// set lines table
+	f.set.mutex.Lock()
+	f.lines = lines
+	f.set.mutex.Unlock()
+}
+
+// A lineInfo object describes alternative file and line number
+// information (such as provided via a //line comment in a .go
+// file) for a given file offset.
+type lineInfo struct {
+	// fields are exported to make them accessible to gob
+	Offset   int
+	Filename string
+	Line     int
+}
+
+// AddLineInfo adds alternative file and line number information for
+// a given file offset. The offset must be larger than the offset for
+// the previously added alternative line info and smaller than the
+// file size; otherwise the information is ignored.
+//
+// AddLineInfo is typically used to register alternative position
+// information for //line filename:line comments in source files.
+//
+func (f *File) AddLineInfo(offset int, filename string, line int) {
+	x := index(offset)
+	f.set.mutex.Lock()
+	if i := len(f.infos); i == 0 || index(f.infos[i-1].Offset) < x && x < f.size {
+		f.infos = append(f.infos, lineInfo{offset, filename, line})
+	}
+	f.set.mutex.Unlock()
+}
+
+// Pos returns the Pos value for the given file offset;
+// the offset must be <= f.Size().
+// f.Pos(f.Offset(p)) == p.
+//
+func (f *File) Pos(offset int, rel RelPos) Pos {
+	if index(offset) > f.size {
+		panic("illegal file offset")
+	}
+	return toPos(f.base+index(offset)) + Pos(rel)
+}
+
+// Offset returns the offset for the given file position p;
+// p must be a valid Pos value in that file.
+// f.Offset(f.Pos(offset)) == offset.
+//
+func (f *File) Offset(p Pos) int {
+	x := p.index()
+	if x < f.base || x > f.base+index(f.size) {
+		panic("illegal Pos value")
+	}
+	return int(x - f.base)
+}
+
+// Line returns the line number for the given file position p;
+// p must be a Pos value in that file or NoPos.
+//
+func (f *File) Line(p Pos) int {
+	return f.Position(p).Line
+}
+
+func searchLineInfos(a []lineInfo, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].Offset > x }) - 1
+}
+
+// unpack returns the filename and line and column number for a file offset.
+// If adjusted is set, unpack will return the filename and line information
+// possibly adjusted by //line comments; otherwise those comments are ignored.
+//
+func (f *File) unpack(offset index, adjusted bool) (filename string, line, column int) {
+	filename = f.name
+	if i := searchInts(f.lines, offset); i >= 0 {
+		line, column = int(i+1), int(offset-f.lines[i]+1)
+	}
+	if adjusted && len(f.infos) > 0 {
+		// almost no files have extra line infos
+		if i := searchLineInfos(f.infos, int(offset)); i >= 0 {
+			alt := &f.infos[i]
+			filename = alt.Filename
+			if i := searchInts(f.lines, index(alt.Offset)); i >= 0 {
+				line += alt.Line - i - 1
+			}
+		}
+	}
+	return
+}
+
+func (f *File) position(p Pos, adjusted bool) (pos Position) {
+	offset := p.index() - f.base
+	pos.Offset = int(offset)
+	pos.Filename, pos.Line, pos.Column = f.unpack(offset, adjusted)
+	return
+}
+
+// PositionFor returns the Position value for the given file position p.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in f or NoPos.
+//
+func (f *File) PositionFor(p Pos, adjusted bool) (pos Position) {
+	x := p.index()
+	if p != NoPos {
+		if x < f.base || x > f.base+f.size {
+			panic("illegal Pos value")
+		}
+		pos = f.position(p, adjusted)
+	}
+	return
+}
+
+// Position returns the Position value for the given file position p.
+// Calling f.Position(p) is equivalent to calling f.PositionFor(p, true).
+//
+func (f *File) Position(p Pos) (pos Position) {
+	return f.PositionFor(p, true)
+}
+
+// A FileSet represents a set of source files.
+// Methods of file sets are synchronized; multiple goroutines
+// may invoke them concurrently.
+type FileSet struct {
+	mutex sync.RWMutex // protects the file set
+	base  int          // base offset for the next file
+	files []*File      // list of files in the order added to the set
+	last  *File        // cache of last file looked up
+}
+
+// NewFileSet creates a new file set.
+func NewFileSet() *FileSet {
+	return &FileSet{
+		base: 1, // 0 == NoPos
+	}
+}
+
+// Base returns the minimum base offset that must be provided to
+// AddFile when adding the next file.
+func (s *FileSet) Base() int {
+	s.mutex.RLock()
+	b := s.base
+	s.mutex.RUnlock()
+	return b
+
+}
+
+// AddFile adds a new file with a given filename, base offset, and file size
+// to the file set s and returns the file. Multiple files may have the same
+// name. The base offset must not be smaller than the FileSet's Base(), and
+// size must not be negative. As a special case, if a negative base is provided,
+// the current value of the FileSet's Base() is used instead.
+//
+// Adding the file will set the file set's Base() value to base + size + 1
+// as the minimum base value for the next file. The following relationship
+// exists between a Pos value p for a given file offset offs:
+//
+//	int(p) = base + offs
+//
+// with offs in the range [0, size] and thus p in the range [base, base+size].
+// For convenience, File.Pos may be used to create file-specific position
+// values from a file offset.
+func (s *FileSet) AddFile(filename string, base, size int) *File {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	if base < 0 {
+		base = s.base
+	}
+	if base < s.base || size < 0 {
+		panic("illegal base or size")
+	}
+	// base >= s.base && size >= 0
+	f := &File{s, filename, index(base), index(size), []index{0}, nil}
+	base += size + 1 // +1 because EOF also has a position
+	if base < 0 {
+		panic("token.Pos offset overflow (> 2G of source code in file set)")
+	}
+	// add the file to the file set
+	s.base = base
+	s.files = append(s.files, f)
+	s.last = f
+	return f
+}
+
+// Iterate calls f for the files in the file set in the order they were added
+// until f returns false.
+//
+func (s *FileSet) Iterate(f func(*File) bool) {
+	for i := 0; ; i++ {
+		var file *File
+		s.mutex.RLock()
+		if i < len(s.files) {
+			file = s.files[i]
+		}
+		s.mutex.RUnlock()
+		if file == nil || !f(file) {
+			break
+		}
+	}
+}
+
+func searchFiles(a []*File, x index) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].base > x }) - 1
+}
+
+func (s *FileSet) file(p Pos) *File {
+	x := p.index()
+	s.mutex.RLock()
+	// common case: p is in last file
+	if f := s.last; f != nil && f.base <= x && x <= f.base+f.size {
+		s.mutex.RUnlock()
+		return f
+	}
+	// p is not in last file - search all files
+	if i := searchFiles(s.files, x); i >= 0 {
+		f := s.files[i]
+		// f.base <= int(p) by definition of searchFiles
+		if x <= f.base+f.size {
+			s.mutex.RUnlock()
+			s.mutex.Lock()
+			s.last = f // race is ok - s.last is only a cache
+			s.mutex.Unlock()
+			return f
+		}
+	}
+	s.mutex.RUnlock()
+	return nil
+}
+
+// File returns the file that contains the position p.
+// If no such file is found (for instance for p == NoPos),
+// the result is nil.
+//
+func (s *FileSet) File(p Pos) (f *File) {
+	if p.index() != 0 {
+		f = s.file(p)
+	}
+	return
+}
+
+// PositionFor converts a Pos p in the fileset into a Position value.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in s or NoPos.
+//
+func (s *FileSet) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p.index() != 0 {
+		if f := s.file(p); f != nil {
+			s.mutex.RLock()
+			pos = f.position(p, adjusted)
+			s.mutex.RUnlock()
+		}
+	}
+	return
+}
+
+// Position converts a Pos p in the fileset into a Position value.
+// Calling s.Position(p) is equivalent to calling s.PositionFor(p, true).
+//
+func (s *FileSet) Position(p Pos) (pos Position) {
+	return s.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// Helper functions
+
+func searchInts(a []index, x index) int {
+	// This function body is a manually inlined version of:
+	//
+	//   return sort.Search(len(a), func(i int) bool { return a[i] > x }) - 1
+	//
+	// With better compiler optimizations, this may not be needed in the
+	// future, but at the moment this change improves the go/printer
+	// benchmark performance by ~30%. This has a direct impact on the
+	// speed of gofmt and thus seems worthwhile (2011-04-29).
+	// TODO(gri): Remove this when compilers have caught up.
+	i, j := 0, len(a)
+	for i < j {
+		h := i + (j-i)/2 // avoid overflow when computing h
+		// i ≤ h < j
+		if a[h] <= x {
+			i = h + 1
+		} else {
+			j = h
+		}
+	}
+	return i - 1
+}
diff --git a/cue/token/position_test.go b/cue/token/position_test.go
new file mode 100644
index 0000000..906cac0
--- /dev/null
+++ b/cue/token/position_test.go
@@ -0,0 +1,336 @@
+// Copyright 2018 The CUE Authors
+//
+// 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 token
+
+import (
+	"fmt"
+	"math/rand"
+	"sync"
+	"testing"
+)
+
+func checkPos(t *testing.T, msg string, got, want Position) {
+	if got.Filename != want.Filename {
+		t.Errorf("%s: got filename = %q; want %q", msg, got.Filename, want.Filename)
+	}
+	if got.Offset != want.Offset {
+		t.Errorf("%s: got offset = %d; want %d", msg, got.Offset, want.Offset)
+	}
+	if got.Line != want.Line {
+		t.Errorf("%s: got line = %d; want %d", msg, got.Line, want.Line)
+	}
+	if got.Column != want.Column {
+		t.Errorf("%s: got column = %d; want %d", msg, got.Column, want.Column)
+	}
+}
+
+func TestNoPos(t *testing.T) {
+	if NoPos.IsValid() {
+		t.Errorf("NoPos should not be valid")
+	}
+	var fset *FileSet
+	checkPos(t, "nil NoPos", fset.Position(NoPos), Position{})
+	fset = NewFileSet()
+	checkPos(t, "fset NoPos", fset.Position(NoPos), Position{})
+}
+
+var tests = []struct {
+	filename string
+	source   []byte // may be nil
+	size     int
+	lines    []int
+}{
+	{"a", []byte{}, 0, []int{}},
+	{"b", []byte("01234"), 5, []int{0}},
+	{"c", []byte("\n\n\n\n\n\n\n\n\n"), 9, []int{0, 1, 2, 3, 4, 5, 6, 7, 8}},
+	{"d", nil, 100, []int{0, 5, 10, 20, 30, 70, 71, 72, 80, 85, 90, 99}},
+	{"e", nil, 777, []int{0, 80, 100, 120, 130, 180, 267, 455, 500, 567, 620}},
+	{"f", []byte("package p\n\nimport \"fmt\""), 23, []int{0, 10, 11}},
+	{"g", []byte("package p\n\nimport \"fmt\"\n"), 24, []int{0, 10, 11}},
+	{"h", []byte("package p\n\nimport \"fmt\"\n "), 25, []int{0, 10, 11, 24}},
+}
+
+func linecol(lines []int, offs int) (int, int) {
+	prevLineOffs := 0
+	for line, lineOffs := range lines {
+		if offs < lineOffs {
+			return line, offs - prevLineOffs + 1
+		}
+		prevLineOffs = lineOffs
+	}
+	return len(lines), offs - prevLineOffs + 1
+}
+
+func verifyPositions(t *testing.T, fset *FileSet, f *File, lines []int) {
+	for offs := 0; offs < f.Size(); offs++ {
+		p := f.Pos(offs, 0)
+		offs2 := f.Offset(p)
+		if offs2 != offs {
+			t.Errorf("%s, Offset: got offset %d; want %d", f.Name(), offs2, offs)
+		}
+		line, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs, 0)), Position{f.Name(), offs, line, col})
+		checkPos(t, msg, fset.Position(p), Position{f.Name(), offs, line, col})
+	}
+}
+
+func makeTestSource(size int, lines []int) []byte {
+	src := make([]byte, size)
+	for _, offs := range lines {
+		if offs > 0 {
+			src[offs-1] = '\n'
+		}
+	}
+	return src
+}
+
+func TestPositions(t *testing.T) {
+	const delta = 7 // a non-zero base offset increment
+	fset := NewFileSet()
+	for _, test := range tests {
+		// verify consistency of test case
+		if test.source != nil && len(test.source) != test.size {
+			t.Errorf("%s: inconsistent test case: got file size %d; want %d", test.filename, len(test.source), test.size)
+		}
+
+		// add file and verify name and size
+		f := fset.AddFile(test.filename, fset.Base()+delta, test.size)
+		if f.Name() != test.filename {
+			t.Errorf("got filename %q; want %q", f.Name(), test.filename)
+		}
+		if f.Size() != test.size {
+			t.Errorf("%s: got file size %d; want %d", f.Name(), f.Size(), test.size)
+		}
+		if fset.File(f.Pos(0, 0)) != f {
+			t.Errorf("%s: f.Pos(0, 0) was not found in f", f.Name())
+		}
+
+		// add lines individually and verify all positions
+		for i, offset := range test.lines {
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			// adding the same offset again should be ignored
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got unchanged line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			verifyPositions(t, fset, f, test.lines[0:i+1])
+		}
+
+		// add lines with SetLines and verify all positions
+		if ok := f.SetLines(test.lines); !ok {
+			t.Errorf("%s: SetLines failed", f.Name())
+		}
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLines: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+
+		// add lines with SetLinesForContent and verify all positions
+		src := test.source
+		if src == nil {
+			// no test source available - create one from scratch
+			src = makeTestSource(test.size, test.lines)
+		}
+		f.SetLinesForContent(src)
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLinesForContent: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+	}
+}
+
+func TestLineInfo(t *testing.T) {
+	fset := NewFileSet()
+	f := fset.AddFile("foo", fset.Base(), 500)
+	lines := []int{0, 42, 77, 100, 210, 220, 277, 300, 333, 401}
+	// add lines individually and provide alternative line information
+	for _, offs := range lines {
+		f.AddLine(offs)
+		f.AddLineInfo(offs, "bar", 42)
+	}
+	// verify positions for all offsets
+	for offs := 0; offs <= f.Size(); offs++ {
+		p := f.Pos(offs, 0)
+		_, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs, 0)), Position{"bar", offs, 42, col})
+		checkPos(t, msg, fset.Position(p), Position{"bar", offs, 42, col})
+	}
+}
+
+func TestFiles(t *testing.T) {
+	fset := NewFileSet()
+	for i, test := range tests {
+		base := fset.Base()
+		if i%2 == 1 {
+			// Setting a negative base is equivalent to
+			// fset.Base(), so test some of each.
+			base = -1
+		}
+		fset.AddFile(test.filename, base, test.size)
+		j := 0
+		fset.Iterate(func(f *File) bool {
+			if f.Name() != tests[j].filename {
+				t.Errorf("got filename = %s; want %s", f.Name(), tests[j].filename)
+			}
+			j++
+			return true
+		})
+		if j != i+1 {
+			t.Errorf("got %d files; want %d", j, i+1)
+		}
+	}
+}
+
+// FileSet.File should return nil if Pos is past the end of the FileSet.
+func TestFileSetPastEnd(t *testing.T) {
+	fset := NewFileSet()
+	for _, test := range tests {
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	if f := fset.File(toPos(index(fset.Base()))); f != nil {
+		t.Errorf("got %v, want nil", f)
+	}
+}
+
+func TestFileSetCacheUnlikely(t *testing.T) {
+	fset := NewFileSet()
+	offsets := make(map[string]index)
+	for _, test := range tests {
+		offsets[test.filename] = index(fset.Base())
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	for file, pos := range offsets {
+		f := fset.File(toPos(pos))
+		if f.Name() != file {
+			t.Errorf("got %q at position %d, want %q", f.Name(), pos, file)
+		}
+	}
+}
+
+// issue 4345. Test that concurrent use of FileSet.Pos does not trigger a
+// race in the FileSet position cache.
+func TestFileSetRace(t *testing.T) {
+	fset := NewFileSet()
+	for i := 0; i < 100; i++ {
+		fset.AddFile(fmt.Sprintf("file-%d", i), fset.Base(), 1031)
+	}
+	max := int32(fset.Base())
+	var stop sync.WaitGroup
+	r := rand.New(rand.NewSource(7))
+	for i := 0; i < 2; i++ {
+		r := rand.New(rand.NewSource(r.Int63()))
+		stop.Add(1)
+		go func() {
+			for i := 0; i < 1000; i++ {
+				fset.Position(Pos(r.Int31n(max)))
+			}
+			stop.Done()
+		}()
+	}
+	stop.Wait()
+}
+
+// issue 16548. Test that concurrent use of File.AddLine and FileSet.PositionFor
+// does not trigger a race in the FileSet position cache.
+func TestFileSetRace2(t *testing.T) {
+	const N = 1e3
+	var (
+		fset = NewFileSet()
+		file = fset.AddFile("", -1, N)
+		ch   = make(chan int, 2)
+	)
+
+	go func() {
+		for i := 0; i < N; i++ {
+			file.AddLine(i)
+		}
+		ch <- 1
+	}()
+
+	go func() {
+		pos := file.Pos(0, 0)
+		for i := 0; i < N; i++ {
+			fset.PositionFor(pos, false)
+		}
+		ch <- 1
+	}()
+
+	<-ch
+	<-ch
+}
+
+func TestPositionFor(t *testing.T) {
+	src := []byte(`
+foo
+b
+ar
+//line :100
+foobar
+//line bar:3
+done
+`)
+
+	const filename = "foo"
+	fset := NewFileSet()
+	f := fset.AddFile(filename, fset.Base(), len(src))
+	f.SetLinesForContent(src)
+
+	// verify position info
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(int(offs), 0), false)
+		got2 := f.PositionFor(f.Pos(int(offs), 0), true)
+		got3 := f.Position(f.Pos(int(offs), 0))
+		want := Position{filename, int(offs), i + 1, 1}
+		checkPos(t, "1. PositionFor unadjusted", got1, want)
+		checkPos(t, "1. PositionFor adjusted", got2, want)
+		checkPos(t, "1. Position", got3, want)
+	}
+
+	// manually add //line info on lines l1, l2
+	const l1, l2 = 5, 7
+	f.AddLineInfo(int(f.lines[l1-1]), "", 100)
+	f.AddLineInfo(int(f.lines[l2-1]), "bar", 3)
+
+	// unadjusted position info must remain unchanged
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(int(offs), 0), false)
+		want := Position{filename, int(offs), i + 1, 1}
+		checkPos(t, "2. PositionFor unadjusted", got1, want)
+	}
+
+	// adjusted position info should have changed
+	for i, offs := range f.lines {
+		got2 := f.PositionFor(f.Pos(int(offs), 0), true)
+		got3 := f.Position(f.Pos(int(offs), 0))
+		want := Position{filename, int(offs), i + 1, 1}
+		// manually compute wanted filename and line
+		line := want.Line
+		if i+1 >= l1 {
+			want.Filename = ""
+			want.Line = line - l1 + 100
+		}
+		if i+1 >= l2 {
+			want.Filename = "bar"
+			want.Line = line - l2 + 3
+		}
+		checkPos(t, "3. PositionFor adjusted", got2, want)
+		checkPos(t, "3. Position", got3, want)
+	}
+}
diff --git a/cue/token/token.go b/cue/token/token.go
new file mode 100644
index 0000000..ef50c34
--- /dev/null
+++ b/cue/token/token.go
@@ -0,0 +1,263 @@
+// Copyright 2018 The CUE Authors
+//
+// 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 token defines constants representing the lexical tokens of the Go
+// programming language and basic operations on tokens (printing, predicates).
+package token // import "cuelang.org/go/cue/token"
+
+import "strconv"
+
+// Token is the set of lexical tokens of the CUE configuration language.
+type Token int
+
+// The list of tokens.
+const (
+	// Special tokens
+	ILLEGAL Token = iota
+	EOF
+	COMMENT
+
+	literalBeg
+	// Identifiers and basic type literals
+	// (these tokens stand for classes of literals)
+	IDENT // main, _tmp
+	INT   // 12_345Mi, 0700, 0xdeadbeef, 1.2M
+	FLOAT // 123.45,
+	// DURATION // 3m4s TODO
+	STRING        // "abc"
+	INTERPOLATION // a part of a template string, e.g. `"age: \(`
+	BOTTOM        // _|_
+
+	literalEnd
+
+	operatorBeg
+	// Operators and delimiters
+	ADD // +
+	SUB // -
+	MUL // *
+	POW // ^
+	QUO // /
+	REM // %
+
+	IQUO // quo
+	IREM // rem
+	IDIV // div
+	IMOD // mod
+
+	UNIFY       // &
+	DISJUNCTION // |
+
+	LAND // &&
+	LOR  // ||
+
+	BIND   // =
+	EQL    // ==
+	LSS    // <
+	GTR    // >
+	NOT    // !
+	ARROW  // <-
+	LAMBDA // ->
+
+	NEQ // !=
+	LEQ // <=
+	GEQ // >=
+
+	LPAREN   // (
+	LBRACK   // [
+	LBRACE   // {
+	COMMA    // ,
+	PERIOD   // .
+	RANGE    // ..
+	ELLIPSIS // ...
+
+	RPAREN    // )
+	RBRACK    // ]
+	RBRACE    // }
+	SEMICOLON // :
+	COLON     // :
+	operatorEnd
+
+	keywordBeg
+
+	IF
+	FOR
+	IN
+	LET
+
+	TRUE
+	FALSE
+	NULL
+
+	keywordEnd
+)
+
+var tokens = [...]string{
+	ILLEGAL: "ILLEGAL",
+
+	EOF:     "EOF",
+	COMMENT: "COMMENT",
+
+	IDENT: "IDENT",
+	INT:   "INT",
+	FLOAT: "FLOAT",
+	// DURATION:      "DURATION", // TODO
+	STRING:        "STRING",
+	INTERPOLATION: "INTERPOLATION",
+
+	ADD: "+",
+	SUB: "-",
+	MUL: "*",
+	POW: "^",
+	QUO: "/",
+	REM: "%",
+
+	IQUO: "quo",
+	IREM: "rem",
+	IDIV: "div",
+	IMOD: "mod",
+
+	UNIFY:       "&",
+	DISJUNCTION: "|",
+
+	LAND: "&&",
+	LOR:  "||",
+
+	BIND:   "=",
+	EQL:    "==",
+	LSS:    "<",
+	GTR:    ">",
+	NOT:    "!",
+	ARROW:  "<-",
+	LAMBDA: "->",
+
+	NEQ: "!=",
+	LEQ: "<=",
+	GEQ: ">=",
+
+	LPAREN:   "(",
+	LBRACK:   "[",
+	LBRACE:   "{",
+	COMMA:    ",",
+	PERIOD:   ".",
+	RANGE:    "..",
+	ELLIPSIS: "...",
+
+	RPAREN:    ")",
+	RBRACK:    "]",
+	RBRACE:    "}",
+	SEMICOLON: ";",
+	COLON:     ":",
+
+	BOTTOM: "_|_",
+
+	FALSE: "false",
+	TRUE:  "true",
+	NULL:  "null",
+
+	FOR: "for",
+	IF:  "if",
+	IN:  "in",
+	LET: "let",
+}
+
+// String returns the string corresponding to the token tok.
+// For operators, delimiters, and keywords the string is the actual
+// token character sequence (e.g., for the token ADD, the string is
+// "+"). For all other tokens the string corresponds to the token
+// constant name (e.g. for the token IDENT, the string is "IDENT").
+func (tok Token) String() string {
+	s := ""
+	if 0 <= tok && tok < Token(len(tokens)) {
+		s = tokens[tok]
+	}
+	if s == "" {
+		s = "token(" + strconv.Itoa(int(tok)) + ")"
+	}
+	return s
+}
+
+// A set of constants for precedence-based expression parsing.
+// Non-operators have lowest precedence, followed by operators
+// starting with precedence 1 up to unary operators. The highest
+// precedence serves as "catch-all" precedence for selector,
+// indexing, and other operator and delimiter tokens.
+const (
+	LowestPrec  = lowestPrec
+	UnaryPrec   = unaryPrec
+	HighestPrec = highestPrec
+)
+
+const (
+	lowestPrec  = 0 // non-operators
+	unaryPrec   = 8
+	highestPrec = 9
+)
+
+// Precedence returns the operator precedence of the binary
+// operator op. If op is not a binary operator, the result
+// is LowestPrecedence.
+//
+func (tok Token) Precedence() int {
+	switch tok {
+	case DISJUNCTION:
+		return 1
+	case UNIFY:
+		return 2
+	case LOR:
+		return 3
+	case LAND:
+		return 4
+	case EQL, NEQ, LSS, LEQ, GTR, GEQ:
+		return 5
+	case ADD, SUB:
+		return 6
+	case MUL, QUO, REM, IDIV, IMOD, IQUO, IREM:
+		return 7
+	case RANGE:
+		return 8
+	}
+	return lowestPrec
+}
+
+var keywords map[string]Token
+
+func init() {
+	keywords = make(map[string]Token)
+	for i := keywordBeg + 1; i < keywordEnd; i++ {
+		keywords[tokens[i]] = i
+	}
+}
+
+// Lookup maps an identifier to its keyword token or IDENT (if not a keyword).
+//
+func Lookup(ident string) Token {
+	if tok, isKeyword := keywords[ident]; isKeyword {
+		return tok
+	}
+	return IDENT
+}
+
+// Predicates
+
+// IsLiteral returns true for tokens corresponding to identifiers
+// and basic type literals; it returns false otherwise.
+func (tok Token) IsLiteral() bool { return literalBeg < tok && tok < literalEnd }
+
+// IsOperator returns true for tokens corresponding to operators and
+// delimiters; it returns false otherwise.
+func (tok Token) IsOperator() bool { return operatorBeg < tok && tok < operatorEnd }
+
+// IsKeyword returns true for tokens corresponding to keywords;
+// it returns false otherwise.
+func (tok Token) IsKeyword() bool { return keywordBeg < tok && tok < keywordEnd }