9 files changed, 364 insertions, 277 deletions

diff --git a/src/pkg/container/heap/example_intheap_test.go b/src/pkg/container/heap/example_intheap_test.go
new file mode 100644
index 000000000..e718cbc58
--- /dev/null
+++ b/src/pkg/container/heap/example_intheap_test.go
@@ -0,0 +1,43 @@
+// Copyright 2012 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.
+
+// This example demonstrates an integer heap built using the heap interface.
+package heap_test
+
+import (
+	"container/heap"
+	"fmt"
+)
+
+// An IntHeap is a min-heap of ints.
+type IntHeap []int
+
+func (h IntHeap) Len() int           { return len(h) }
+func (h IntHeap) Less(i, j int) bool { return h[i] < h[j] }
+func (h IntHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
+
+func (h *IntHeap) Push(x interface{}) {
+	// Push and Pop use pointer receivers because they modify the slice's length,
+	// not just its contents.
+	*h = append(*h, x.(int))
+}
+
+func (h *IntHeap) Pop() interface{} {
+	old := *h
+	n := len(old)
+	x := old[n-1]
+	*h = old[0 : n-1]
+	return x
+}
+
+// This example inserts several ints into an IntHeap and removes them in order of priority.
+func Example_intHeap() {
+	h := &IntHeap{2, 1, 5}
+	heap.Init(h)
+	heap.Push(h, 3)
+	for h.Len() > 0 {
+		fmt.Printf("%d ", heap.Pop(h))
+	}
+	// Output: 1 2 3 5
+}
diff --git a/src/pkg/container/heap/example_pq_test.go b/src/pkg/container/heap/example_pq_test.go
new file mode 100644
index 000000000..8cbeb8d70
--- /dev/null
+++ b/src/pkg/container/heap/example_pq_test.go
@@ -0,0 +1,95 @@
+// Copyright 2012 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.
+
+// This example demonstrates a priority queue built using the heap interface.
+package heap_test
+
+import (
+	"container/heap"
+	"fmt"
+)
+
+// An Item is something we manage in a priority queue.
+type Item struct {
+	value    string // The value of the item; arbitrary.
+	priority int    // The priority of the item in the queue.
+	// The index is needed by update and is maintained by the heap.Interface methods.
+	index int // The index of the item in the heap.
+}
+
+// A PriorityQueue implements heap.Interface and holds Items.
+type PriorityQueue []*Item
+
+func (pq PriorityQueue) Len() int { return len(pq) }
+
+func (pq PriorityQueue) Less(i, j int) bool {
+	// We want Pop to give us the highest, not lowest, priority so we use greater than here.
+	return pq[i].priority > pq[j].priority
+}
+
+func (pq PriorityQueue) Swap(i, j int) {
+	pq[i], pq[j] = pq[j], pq[i]
+	pq[i].index = i
+	pq[j].index = j
+}
+
+func (pq *PriorityQueue) Push(x interface{}) {
+	n := len(*pq)
+	item := x.(*Item)
+	item.index = n
+	*pq = append(*pq, item)
+}
+
+func (pq *PriorityQueue) Pop() interface{} {
+	old := *pq
+	n := len(old)
+	item := old[n-1]
+	item.index = -1 // for safety
+	*pq = old[0 : n-1]
+	return item
+}
+
+// update modifies the priority and value of an Item in the queue.
+func (pq *PriorityQueue) update(item *Item, value string, priority int) {
+	heap.Remove(pq, item.index)
+	item.value = value
+	item.priority = priority
+	heap.Push(pq, item)
+}
+
+// This example inserts some items into a PriorityQueue, manipulates an item,
+// and then removes the items in priority order.
+func Example_priorityQueue() {
+	// Some items and their priorities.
+	items := map[string]int{
+		"banana": 3, "apple": 2, "pear": 4,
+	}
+
+	// Create a priority queue and put the items in it.
+	pq := &PriorityQueue{}
+	heap.Init(pq)
+	for value, priority := range items {
+		item := &Item{
+			value:    value,
+			priority: priority,
+		}
+		heap.Push(pq, item)
+	}
+
+	// Insert a new item and then modify its priority.
+	item := &Item{
+		value:    "orange",
+		priority: 1,
+	}
+	heap.Push(pq, item)
+	pq.update(item, item.value, 5)
+
+	// Take the items out; they arrive in decreasing priority order.
+	for pq.Len() > 0 {
+		item := heap.Pop(pq).(*Item)
+		fmt.Printf("%.2d:%s ", item.priority, item.value)
+	}
+	// Output:
+	// 05:orange 04:pear 03:banana 02:apple
+}
diff --git a/src/pkg/container/heap/example_test.go b/src/pkg/container/heap/example_test.go
deleted file mode 100644
index 2050bc835..000000000
--- a/src/pkg/container/heap/example_test.go
+++ /dev/null
@@ -1,105 +0,0 @@
-// Copyright 2012 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.
-
-// This example demonstrates a priority queue built using the heap interface.
-package heap_test
-
-import (
-	"container/heap"
-	"fmt"
-)
-
-// An Item is something we manage in a priority queue.
-type Item struct {
-	value    string // The value of the item; arbitrary.
-	priority int    // The priority of the item in the queue.
-	// The index is needed by changePriority and is maintained by the heap.Interface methods.
-	index int // The index of the item in the heap.
-}
-
-// A PriorityQueue implements heap.Interface and holds Items.
-type PriorityQueue []*Item
-
-func (pq PriorityQueue) Len() int { return len(pq) }
-
-func (pq PriorityQueue) Less(i, j int) bool {
-	// We want Pop to give us the highest, not lowest, priority so we use greater than here.
-	return pq[i].priority > pq[j].priority
-}
-
-func (pq PriorityQueue) Swap(i, j int) {
-	pq[i], pq[j] = pq[j], pq[i]
-	pq[i].index = i
-	pq[j].index = j
-}
-
-func (pq *PriorityQueue) Push(x interface{}) {
-	// Push and Pop use pointer receivers because they modify the slice's length,
-	// not just its contents.
-	// To simplify indexing expressions in these methods, we save a copy of the
-	// slice object. We could instead write (*pq)[i].
-	a := *pq
-	n := len(a)
-	a = a[0 : n+1]
-	item := x.(*Item)
-	item.index = n
-	a[n] = item
-	*pq = a
-}
-
-func (pq *PriorityQueue) Pop() interface{} {
-	a := *pq
-	n := len(a)
-	item := a[n-1]
-	item.index = -1 // for safety
-	*pq = a[0 : n-1]
-	return item
-}
-
-// update is not used by the example but shows how to take the top item from
-// the queue, update its priority and value, and put it back.
-func (pq *PriorityQueue) update(value string, priority int) {
-	item := heap.Pop(pq).(*Item)
-	item.value = value
-	item.priority = priority
-	heap.Push(pq, item)
-}
-
-// changePriority is not used by the example but shows how to change the
-// priority of an arbitrary item.
-func (pq *PriorityQueue) changePriority(item *Item, priority int) {
-	heap.Remove(pq, item.index)
-	item.priority = priority
-	heap.Push(pq, item)
-}
-
-// This example pushes 10 items into a PriorityQueue and takes them out in
-// order of priority.
-func Example() {
-	const nItem = 10
-	// Random priorities for the items (a permutation of 0..9, times 11)).
-	priorities := [nItem]int{
-		77, 22, 44, 55, 11, 88, 33, 99, 00, 66,
-	}
-	values := [nItem]string{
-		"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine",
-	}
-	// Create a priority queue and put some items in it.
-	pq := make(PriorityQueue, 0, nItem)
-	for i := 0; i < cap(pq); i++ {
-		item := &Item{
-			value:    values[i],
-			priority: priorities[i],
-		}
-		heap.Push(&pq, item)
-	}
-	// Take the items out; should arrive in decreasing priority order.
-	// For example, the highest priority (99) is the seventh item, so output starts with 99:"seven".
-	for i := 0; i < nItem; i++ {
-		item := heap.Pop(&pq).(*Item)
-		fmt.Printf("%.2d:%s ", item.priority, item.value)
-	}
-	// Output:
-	// 99:seven 88:five 77:zero 66:nine 55:three 44:two 33:six 22:one 11:four 00:eight
-}
diff --git a/src/pkg/container/heap/heap.go b/src/pkg/container/heap/heap.go
index 67018e6ba..c37e50e3c 100644
--- a/src/pkg/container/heap/heap.go
+++ b/src/pkg/container/heap/heap.go
@@ -4,13 +4,13 @@
 
 // Package heap provides heap operations for any type that implements
 // heap.Interface. A heap is a tree with the property that each node is the
-// highest-valued node in its subtree.
+// minimum-valued node in its subtree.
 //
 // A heap is a common way to implement a priority queue. To build a priority
 // queue, implement the Heap interface with the (negative) priority as the
 // ordering for the Less method, so Push adds items while Pop removes the
 // highest-priority item from the queue. The Examples include such an
-// implementation; the file example_test.go has the complete source.
+// implementation; the file example_pq_test.go has the complete source.
 //
 package heap
 
@@ -79,7 +79,7 @@ func Remove(h Interface, i int) interface{} {
 func up(h Interface, j int) {
 	for {
 		i := (j - 1) / 2 // parent
-		if i == j || h.Less(i, j) {
+		if i == j || !h.Less(j, i) {
 			break
 		}
 		h.Swap(i, j)
@@ -90,14 +90,14 @@ func up(h Interface, j int) {
 func down(h Interface, i, n int) {
 	for {
 		j1 := 2*i + 1
-		if j1 >= n {
+		if j1 >= n || j1 < 0 { // j1 < 0 after int overflow
 			break
 		}
 		j := j1 // left child
 		if j2 := j1 + 1; j2 < n && !h.Less(j1, j2) {
 			j = j2 // = 2*i + 2  // right child
 		}
-		if h.Less(i, j) {
+		if !h.Less(j, i) {
 			break
 		}
 		h.Swap(i, j)
diff --git a/src/pkg/container/heap/heap_test.go b/src/pkg/container/heap/heap_test.go
index cb31ef6d3..274d587d8 100644
--- a/src/pkg/container/heap/heap_test.go
+++ b/src/pkg/container/heap/heap_test.go
@@ -2,10 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package heap_test
+package heap
 
 import (
-	. "container/heap"
 	"testing"
 )
 
@@ -170,3 +169,16 @@ func TestRemove2(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkDup(b *testing.B) {
+	const n = 10000
+	h := make(myHeap, n)
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < n; j++ {
+			Push(&h, 0) // all elements are the same
+		}
+		for h.Len() > 0 {
+			Pop(&h)
+		}
+	}
+}
diff --git a/src/pkg/container/list/example_test.go b/src/pkg/container/list/example_test.go
new file mode 100644
index 000000000..7361212d7
--- /dev/null
+++ b/src/pkg/container/list/example_test.go
@@ -0,0 +1,30 @@
+// Copyright 2013 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 list_test
+
+import (
+	"container/list"
+	"fmt"
+)
+
+func Example() {
+	// Create a new list and put some numbers in it.
+	l := list.New()
+	e4 := l.PushBack(4)
+	e1 := l.PushFront(1)
+	l.InsertBefore(3, e4)
+	l.InsertAfter(2, e1)
+
+	// Iterate through list and and print its contents.
+	for e := l.Front(); e != nil; e = e.Next() {
+		fmt.Println(e.Value)
+	}
+
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 4
+}
diff --git a/src/pkg/container/list/list.go b/src/pkg/container/list/list.go
index a3fd4b39f..562a5badb 100644
--- a/src/pkg/container/list/list.go
+++ b/src/pkg/container/list/list.go
@@ -11,201 +11,187 @@
 //
 package list
 
-// Element is an element in the linked list.
+// Element is an element of a linked list.
 type Element struct {
 	// Next and previous pointers in the doubly-linked list of elements.
-	// The front of the list has prev = nil, and the back has next = nil.
+	// To simplify the implementation, internally a list l is implemented
+	// as a ring, such that &l.root is both the next element of the last
+	// list element (l.Back()) and the previous element of the first list
+	// element (l.Front()).
 	next, prev *Element
 
 	// The list to which this element belongs.
 	list *List
 
-	// The contents of this list element.
+	// The value stored with this element.
 	Value interface{}
 }
 
 // Next returns the next list element or nil.
-func (e *Element) Next() *Element { return e.next }
+func (e *Element) Next() *Element {
+	if p := e.next; p != &e.list.root {
+		return p
+	}
+	return nil
+}
 
 // Prev returns the previous list element or nil.
-func (e *Element) Prev() *Element { return e.prev }
+func (e *Element) Prev() *Element {
+	if p := e.prev; p != &e.list.root {
+		return p
+	}
+	return nil
+}
 
 // List represents a doubly linked list.
 // The zero value for List is an empty list ready to use.
 type List struct {
-	front, back *Element
-	len         int
+	root Element // sentinel list element, only &root, root.prev, and root.next are used
+	len  int     // current list length excluding (this) sentinel element
 }
 
-// Init initializes or clears a List.
+// Init initializes or clears list l.
 func (l *List) Init() *List {
-	l.front = nil
-	l.back = nil
+	l.root.next = &l.root
+	l.root.prev = &l.root
 	l.len = 0
 	return l
 }
 
 // New returns an initialized list.
-func New() *List { return new(List) }
-
-// Front returns the first element in the list.
-func (l *List) Front() *Element { return l.front }
+func New() *List { return new(List).Init() }
 
-// Back returns the last element in the list.
-func (l *List) Back() *Element { return l.back }
+// Len returns the number of elements of list l.
+func (l *List) Len() int { return l.len }
 
-// Remove removes the element from the list
-// and returns its Value.
-func (l *List) Remove(e *Element) interface{} {
-	l.remove(e)
-	e.list = nil // do what remove does not
-	return e.Value
+// Front returns the first element of list l or nil
+func (l *List) Front() *Element {
+	if l.len == 0 {
+		return nil
+	}
+	return l.root.next
 }
 
-// remove the element from the list, but do not clear the Element's list field.
-// This is so that other List methods may use remove when relocating Elements
-// without needing to restore the list field.
-func (l *List) remove(e *Element) {
-	if e.list != l {
-		return
-	}
-	if e.prev == nil {
-		l.front = e.next
-	} else {
-		e.prev.next = e.next
-	}
-	if e.next == nil {
-		l.back = e.prev
-	} else {
-		e.next.prev = e.prev
+// Back returns the last element of list l or nil.
+func (l *List) Back() *Element {
+	if l.len == 0 {
+		return nil
 	}
-
-	e.prev = nil
-	e.next = nil
-	l.len--
+	return l.root.prev
 }
 
-func (l *List) insertBefore(e *Element, mark *Element) {
-	if mark.prev == nil {
-		// new front of the list
-		l.front = e
-	} else {
-		mark.prev.next = e
+// lazyInit lazily initializes a zero List value.
+func (l *List) lazyInit() {
+	if l.root.next == nil {
+		l.Init()
 	}
-	e.prev = mark.prev
-	mark.prev = e
-	e.next = mark
-	l.len++
 }
 
-func (l *List) insertAfter(e *Element, mark *Element) {
-	if mark.next == nil {
-		// new back of the list
-		l.back = e
-	} else {
-		mark.next.prev = e
-	}
-	e.next = mark.next
-	mark.next = e
-	e.prev = mark
+// insert inserts e after at, increments l.len, and returns e.
+func (l *List) insert(e, at *Element) *Element {
+	n := at.next
+	at.next = e
+	e.prev = at
+	e.next = n
+	n.prev = e
+	e.list = l
 	l.len++
+	return e
 }
 
-func (l *List) insertFront(e *Element) {
-	if l.front == nil {
-		// empty list
-		l.front, l.back = e, e
-		e.prev, e.next = nil, nil
-		l.len = 1
-		return
-	}
-	l.insertBefore(e, l.front)
+// insertValue is a convenience wrapper for insert(&Element{Value: v}, at).
+func (l *List) insertValue(v interface{}, at *Element) *Element {
+	return l.insert(&Element{Value: v}, at)
 }
 
-func (l *List) insertBack(e *Element) {
-	if l.back == nil {
-		// empty list
-		l.front, l.back = e, e
-		e.prev, e.next = nil, nil
-		l.len = 1
-		return
+// remove removes e from its list, decrements l.len, and returns e.
+func (l *List) remove(e *Element) *Element {
+	e.prev.next = e.next
+	e.next.prev = e.prev
+	e.next = nil // avoid memory leaks
+	e.prev = nil // avoid memory leaks
+	e.list = nil
+	l.len--
+	return e
+}
+
+// Remove removes e from l if e is an element of list l.
+// It returns the element value e.Value.
+func (l *List) Remove(e *Element) interface{} {
+	if e.list == l {
+		// if e.list == l, l must have been initialized when e was inserted
+		// in l or l == nil (e is a zero Element) and l.remove will crash
+		l.remove(e)
 	}
-	l.insertAfter(e, l.back)
+	return e.Value
 }
 
-// PushFront inserts the value at the front of the list and returns a new Element containing the value.
-func (l *List) PushFront(value interface{}) *Element {
-	e := &Element{nil, nil, l, value}
-	l.insertFront(e)
-	return e
+// Pushfront inserts a new element e with value v at the front of list l and returns e.
+func (l *List) PushFront(v interface{}) *Element {
+	l.lazyInit()
+	return l.insertValue(v, &l.root)
 }
 
-// PushBack inserts the value at the back of the list and returns a new Element containing the value.
-func (l *List) PushBack(value interface{}) *Element {
-	e := &Element{nil, nil, l, value}
-	l.insertBack(e)
-	return e
+// PushBack inserts a new element e with value v at the back of list l and returns e.
+func (l *List) PushBack(v interface{}) *Element {
+	l.lazyInit()
+	return l.insertValue(v, l.root.prev)
 }
 
-// InsertBefore inserts the value immediately before mark and returns a new Element containing the value.
-func (l *List) InsertBefore(value interface{}, mark *Element) *Element {
+// InsertBefore inserts a new element e with value v immediately before mark and returns e.
+// If mark is not an element of l, the list is not modified.
+func (l *List) InsertBefore(v interface{}, mark *Element) *Element {
 	if mark.list != l {
 		return nil
 	}
-	e := &Element{nil, nil, l, value}
-	l.insertBefore(e, mark)
-	return e
+	// see comment in List.Remove about initialization of l
+	return l.insertValue(v, mark.prev)
 }
 
-// InsertAfter inserts the value immediately after mark and returns a new Element containing the value.
-func (l *List) InsertAfter(value interface{}, mark *Element) *Element {
+// InsertAfter inserts a new element e with value v immediately after mark and returns e.
+// If mark is not an element of l, the list is not modified.
+func (l *List) InsertAfter(v interface{}, mark *Element) *Element {
 	if mark.list != l {
 		return nil
 	}
-	e := &Element{nil, nil, l, value}
-	l.insertAfter(e, mark)
-	return e
+	// see comment in List.Remove about initialization of l
+	return l.insertValue(v, mark)
 }
 
-// MoveToFront moves the element to the front of the list.
+// MoveToFront moves element e to the front of list l.
+// If e is not an element of l, the list is not modified.
 func (l *List) MoveToFront(e *Element) {
-	if e.list != l || l.front == e {
+	if e.list != l || l.root.next == e {
 		return
 	}
-	l.remove(e)
-	l.insertFront(e)
+	// see comment in List.Remove about initialization of l
+	l.insert(l.remove(e), &l.root)
 }
 
-// MoveToBack moves the element to the back of the list.
+// MoveToBack moves element e to the back of list l.
+// If e is not an element of l, the list is not modified.
 func (l *List) MoveToBack(e *Element) {
-	if e.list != l || l.back == e {
+	if e.list != l || l.root.prev == e {
 		return
 	}
-	l.remove(e)
-	l.insertBack(e)
+	// see comment in List.Remove about initialization of l
+	l.insert(l.remove(e), l.root.prev)
 }
 
-// Len returns the number of elements in the list.
-func (l *List) Len() int { return l.len }
-
-// PushBackList inserts each element of ol at the back of the list.
-func (l *List) PushBackList(ol *List) {
-	last := ol.Back()
-	for e := ol.Front(); e != nil; e = e.Next() {
-		l.PushBack(e.Value)
-		if e == last {
-			break
-		}
+// PushBackList inserts a copy of an other list at the back of list l.
+// The lists l and other may be the same.
+func (l *List) PushBackList(other *List) {
+	l.lazyInit()
+	for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() {
+		l.insertValue(e.Value, l.root.prev)
 	}
 }
 
-// PushFrontList inserts each element of ol at the front of the list. The ordering of the passed list is preserved.
-func (l *List) PushFrontList(ol *List) {
-	first := ol.Front()
-	for e := ol.Back(); e != nil; e = e.Prev() {
-		l.PushFront(e.Value)
-		if e == first {
-			break
-		}
+// PushFrontList inserts a copy of an other list at the front of list l.
+// The lists l and other may be the same.
+func (l *List) PushFrontList(other *List) {
+	l.lazyInit()
+	for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() {
+		l.insertValue(e.Value, &l.root)
 	}
 }
diff --git a/src/pkg/container/list/list_test.go b/src/pkg/container/list/list_test.go
index 1d44ff84e..b4fc77d14 100644
--- a/src/pkg/container/list/list_test.go
+++ b/src/pkg/container/list/list_test.go
@@ -4,65 +4,75 @@
 
 package list
 
-import (
-	"testing"
-)
+import "testing"
+
+func checkListLen(t *testing.T, l *List, len int) bool {
+	if n := l.Len(); n != len {
+		t.Errorf("l.Len() = %d, want %d", n, len)
+		return false
+	}
+	return true
+}
 
 func checkListPointers(t *testing.T, l *List, es []*Element) {
-	if len(es) == 0 {
-		if l.front != nil || l.back != nil {
-			t.Errorf("l.front/l.back = %v/%v should be nil/nil", l.front, l.back)
-		}
+	root := &l.root
+
+	if !checkListLen(t, l, len(es)) {
 		return
 	}
 
-	if l.front != es[0] {
-		t.Errorf("l.front = %v, want %v", l.front, es[0])
-	}
-	if last := es[len(es)-1]; l.back != last {
-		t.Errorf("l.back = %v, want %v", l.back, last)
+	// zero length lists must be the zero value or properly initialized (sentinel circle)
+	if len(es) == 0 {
+		if l.root.next != nil && l.root.next != root || l.root.prev != nil && l.root.prev != root {
+			t.Errorf("l.root.next = %p, l.root.prev = %p; both should both be nil or %p", l.root.next, l.root.prev, root)
+		}
+		return
 	}
+	// len(es) > 0
 
+	// check internal and external prev/next connections
 	for i, e := range es {
-		var e_prev, e_next *Element = nil, nil
+		prev := root
+		Prev := (*Element)(nil)
 		if i > 0 {
-			e_prev = es[i-1]
+			prev = es[i-1]
+			Prev = prev
+		}
+		if p := e.prev; p != prev {
+			t.Errorf("elt[%d](%p).prev = %p, want %p", i, e, p, prev)
+		}
+		if p := e.Prev(); p != Prev {
+			t.Errorf("elt[%d](%p).Prev() = %p, want %p", i, e, p, Prev)
 		}
+
+		next := root
+		Next := (*Element)(nil)
 		if i < len(es)-1 {
-			e_next = es[i+1]
+			next = es[i+1]
+			Next = next
 		}
-		if e.prev != e_prev {
-			t.Errorf("elt #%d (%v) has prev=%v, want %v", i, e, e.prev, e_prev)
+		if n := e.next; n != next {
+			t.Errorf("elt[%d](%p).next = %p, want %p", i, e, n, next)
 		}
-		if e.next != e_next {
-			t.Errorf("elt #%d (%v) has next=%v, want %v", i, e, e.next, e_next)
+		if n := e.Next(); n != Next {
+			t.Errorf("elt[%d](%p).Next() = %p, want %p", i, e, n, Next)
 		}
 	}
 }
 
-func checkListLen(t *testing.T, l *List, n int) {
-	if an := l.Len(); an != n {
-		t.Errorf("l.Len() = %d, want %d", an, n)
-	}
-}
-
 func TestList(t *testing.T) {
 	l := New()
 	checkListPointers(t, l, []*Element{})
-	checkListLen(t, l, 0)
 
 	// Single element list
 	e := l.PushFront("a")
-	checkListLen(t, l, 1)
 	checkListPointers(t, l, []*Element{e})
 	l.MoveToFront(e)
 	checkListPointers(t, l, []*Element{e})
 	l.MoveToBack(e)
 	checkListPointers(t, l, []*Element{e})
-	checkListLen(t, l, 1)
 	l.Remove(e)
 	checkListPointers(t, l, []*Element{})
-	checkListLen(t, l, 0)
 
 	// Bigger list
 	e2 := l.PushFront(2)
@@ -70,11 +80,9 @@ func TestList(t *testing.T) {
 	e3 := l.PushBack(3)
 	e4 := l.PushBack("banana")
 	checkListPointers(t, l, []*Element{e1, e2, e3, e4})
-	checkListLen(t, l, 4)
 
 	l.Remove(e2)
 	checkListPointers(t, l, []*Element{e1, e3, e4})
-	checkListLen(t, l, 3)
 
 	l.MoveToFront(e3) // move from middle
 	checkListPointers(t, l, []*Element{e3, e1, e4})
@@ -121,7 +129,7 @@ func TestList(t *testing.T) {
 		}
 	}
 	if sum != 4 {
-		t.Errorf("sum over l.Iter() = %d, want 4", sum)
+		t.Errorf("sum over l = %d, want 4", sum)
 	}
 
 	// Clear all elements by iterating
@@ -131,19 +139,18 @@ func TestList(t *testing.T) {
 		l.Remove(e)
 	}
 	checkListPointers(t, l, []*Element{})
-	checkListLen(t, l, 0)
 }
 
 func checkList(t *testing.T, l *List, es []interface{}) {
-	if l.Len() != len(es) {
-		t.Errorf("list has len=%v, want %v", l.Len(), len(es))
+	if !checkListLen(t, l, len(es)) {
 		return
 	}
+
 	i := 0
 	for e := l.Front(); e != nil; e = e.Next() {
 		le := e.Value.(int)
 		if le != es[i] {
-			t.Errorf("elt #%d has value=%v, want %v", i, le, es[i])
+			t.Errorf("elt[%d].Value = %v, want %v", i, le, es[i])
 		}
 		i++
 	}
@@ -202,8 +209,27 @@ func TestRemove(t *testing.T) {
 	e := l.Front()
 	l.Remove(e)
 	checkListPointers(t, l, []*Element{e2})
-	checkListLen(t, l, 1)
 	l.Remove(e)
 	checkListPointers(t, l, []*Element{e2})
-	checkListLen(t, l, 1)
+}
+
+func TestIssue4103(t *testing.T) {
+	l1 := New()
+	l1.PushBack(1)
+	l1.PushBack(2)
+
+	l2 := New()
+	l2.PushBack(3)
+	l2.PushBack(4)
+
+	e := l1.Front()
+	l2.Remove(e) // l2 should not change because e is not an element of l2
+	if n := l2.Len(); n != 2 {
+		t.Errorf("l2.Len() = %d, want 2", n)
+	}
+
+	l1.InsertBefore(8, e)
+	if n := l1.Len(); n != 3 {
+		t.Errorf("l1.Len() = %d, want 3", n)
+	}
 }
diff --git a/src/pkg/container/ring/ring.go b/src/pkg/container/ring/ring.go
index 1d96918d3..6d3b3e5b3 100644
--- a/src/pkg/container/ring/ring.go
+++ b/src/pkg/container/ring/ring.go
@@ -74,7 +74,7 @@ func New(n int) *Ring {
 	return r
 }
 
-// Link connects ring r with with ring s such that r.Next()
+// Link connects ring r with ring s such that r.Next()
 // becomes s and returns the original value for r.Next().
 // r must not be empty.
 //