diff --git a/vendor/github.com/google/btree/btree.go b/vendor/github.com/google/btree/btree.go
new file mode 100644
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--- /dev/null
+++ b/vendor/github.com/google/btree/btree.go
@@ -0,0 +1,890 @@
+// Copyright 2014 Google Inc.
+//
+// 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 btree implements in-memory B-Trees of arbitrary degree.
+//
+// btree implements an in-memory B-Tree for use as an ordered data structure.
+// It is not meant for persistent storage solutions.
+//
+// It has a flatter structure than an equivalent red-black or other binary tree,
+// which in some cases yields better memory usage and/or performance.
+// See some discussion on the matter here:
+//   http://google-opensource.blogspot.com/2013/01/c-containers-that-save-memory-and-time.html
+// Note, though, that this project is in no way related to the C++ B-Tree
+// implementation written about there.
+//
+// Within this tree, each node contains a slice of items and a (possibly nil)
+// slice of children.  For basic numeric values or raw structs, this can cause
+// efficiency differences when compared to equivalent C++ template code that
+// stores values in arrays within the node:
+//   * Due to the overhead of storing values as interfaces (each
+//     value needs to be stored as the value itself, then 2 words for the
+//     interface pointing to that value and its type), resulting in higher
+//     memory use.
+//   * Since interfaces can point to values anywhere in memory, values are
+//     most likely not stored in contiguous blocks, resulting in a higher
+//     number of cache misses.
+// These issues don't tend to matter, though, when working with strings or other
+// heap-allocated structures, since C++-equivalent structures also must store
+// pointers and also distribute their values across the heap.
+//
+// This implementation is designed to be a drop-in replacement to gollrb.LLRB
+// trees, (http://github.com/petar/gollrb), an excellent and probably the most
+// widely used ordered tree implementation in the Go ecosystem currently.
+// Its functions, therefore, exactly mirror those of
+// llrb.LLRB where possible.  Unlike gollrb, though, we currently don't
+// support storing multiple equivalent values.
+package btree
+
+import (
+	"fmt"
+	"io"
+	"sort"
+	"strings"
+	"sync"
+)
+
+// Item represents a single object in the tree.
+type Item interface {
+	// Less tests whether the current item is less than the given argument.
+	//
+	// This must provide a strict weak ordering.
+	// If !a.Less(b) && !b.Less(a), we treat this to mean a == b (i.e. we can only
+	// hold one of either a or b in the tree).
+	Less(than Item) bool
+}
+
+const (
+	DefaultFreeListSize = 32
+)
+
+var (
+	nilItems    = make(items, 16)
+	nilChildren = make(children, 16)
+)
+
+// FreeList represents a free list of btree nodes. By default each
+// BTree has its own FreeList, but multiple BTrees can share the same
+// FreeList.
+// Two Btrees using the same freelist are safe for concurrent write access.
+type FreeList struct {
+	mu       sync.Mutex
+	freelist []*node
+}
+
+// NewFreeList creates a new free list.
+// size is the maximum size of the returned free list.
+func NewFreeList(size int) *FreeList {
+	return &FreeList{freelist: make([]*node, 0, size)}
+}
+
+func (f *FreeList) newNode() (n *node) {
+	f.mu.Lock()
+	index := len(f.freelist) - 1
+	if index < 0 {
+		f.mu.Unlock()
+		return new(node)
+	}
+	n = f.freelist[index]
+	f.freelist[index] = nil
+	f.freelist = f.freelist[:index]
+	f.mu.Unlock()
+	return
+}
+
+// freeNode adds the given node to the list, returning true if it was added
+// and false if it was discarded.
+func (f *FreeList) freeNode(n *node) (out bool) {
+	f.mu.Lock()
+	if len(f.freelist) < cap(f.freelist) {
+		f.freelist = append(f.freelist, n)
+		out = true
+	}
+	f.mu.Unlock()
+	return
+}
+
+// ItemIterator allows callers of Ascend* to iterate in-order over portions of
+// the tree.  When this function returns false, iteration will stop and the
+// associated Ascend* function will immediately return.
+type ItemIterator func(i Item) bool
+
+// New creates a new B-Tree with the given degree.
+//
+// New(2), for example, will create a 2-3-4 tree (each node contains 1-3 items
+// and 2-4 children).
+func New(degree int) *BTree {
+	return NewWithFreeList(degree, NewFreeList(DefaultFreeListSize))
+}
+
+// NewWithFreeList creates a new B-Tree that uses the given node free list.
+func NewWithFreeList(degree int, f *FreeList) *BTree {
+	if degree <= 1 {
+		panic("bad degree")
+	}
+	return &BTree{
+		degree: degree,
+		cow:    &copyOnWriteContext{freelist: f},
+	}
+}
+
+// items stores items in a node.
+type items []Item
+
+// insertAt inserts a value into the given index, pushing all subsequent values
+// forward.
+func (s *items) insertAt(index int, item Item) {
+	*s = append(*s, nil)
+	if index < len(*s) {
+		copy((*s)[index+1:], (*s)[index:])
+	}
+	(*s)[index] = item
+}
+
+// removeAt removes a value at a given index, pulling all subsequent values
+// back.
+func (s *items) removeAt(index int) Item {
+	item := (*s)[index]
+	copy((*s)[index:], (*s)[index+1:])
+	(*s)[len(*s)-1] = nil
+	*s = (*s)[:len(*s)-1]
+	return item
+}
+
+// pop removes and returns the last element in the list.
+func (s *items) pop() (out Item) {
+	index := len(*s) - 1
+	out = (*s)[index]
+	(*s)[index] = nil
+	*s = (*s)[:index]
+	return
+}
+
+// truncate truncates this instance at index so that it contains only the
+// first index items. index must be less than or equal to length.
+func (s *items) truncate(index int) {
+	var toClear items
+	*s, toClear = (*s)[:index], (*s)[index:]
+	for len(toClear) > 0 {
+		toClear = toClear[copy(toClear, nilItems):]
+	}
+}
+
+// find returns the index where the given item should be inserted into this
+// list.  'found' is true if the item already exists in the list at the given
+// index.
+func (s items) find(item Item) (index int, found bool) {
+	i := sort.Search(len(s), func(i int) bool {
+		return item.Less(s[i])
+	})
+	if i > 0 && !s[i-1].Less(item) {
+		return i - 1, true
+	}
+	return i, false
+}
+
+// children stores child nodes in a node.
+type children []*node
+
+// insertAt inserts a value into the given index, pushing all subsequent values
+// forward.
+func (s *children) insertAt(index int, n *node) {
+	*s = append(*s, nil)
+	if index < len(*s) {
+		copy((*s)[index+1:], (*s)[index:])
+	}
+	(*s)[index] = n
+}
+
+// removeAt removes a value at a given index, pulling all subsequent values
+// back.
+func (s *children) removeAt(index int) *node {
+	n := (*s)[index]
+	copy((*s)[index:], (*s)[index+1:])
+	(*s)[len(*s)-1] = nil
+	*s = (*s)[:len(*s)-1]
+	return n
+}
+
+// pop removes and returns the last element in the list.
+func (s *children) pop() (out *node) {
+	index := len(*s) - 1
+	out = (*s)[index]
+	(*s)[index] = nil
+	*s = (*s)[:index]
+	return
+}
+
+// truncate truncates this instance at index so that it contains only the
+// first index children. index must be less than or equal to length.
+func (s *children) truncate(index int) {
+	var toClear children
+	*s, toClear = (*s)[:index], (*s)[index:]
+	for len(toClear) > 0 {
+		toClear = toClear[copy(toClear, nilChildren):]
+	}
+}
+
+// node is an internal node in a tree.
+//
+// It must at all times maintain the invariant that either
+//   * len(children) == 0, len(items) unconstrained
+//   * len(children) == len(items) + 1
+type node struct {
+	items    items
+	children children
+	cow      *copyOnWriteContext
+}
+
+func (n *node) mutableFor(cow *copyOnWriteContext) *node {
+	if n.cow == cow {
+		return n
+	}
+	out := cow.newNode()
+	if cap(out.items) >= len(n.items) {
+		out.items = out.items[:len(n.items)]
+	} else {
+		out.items = make(items, len(n.items), cap(n.items))
+	}
+	copy(out.items, n.items)
+	// Copy children
+	if cap(out.children) >= len(n.children) {
+		out.children = out.children[:len(n.children)]
+	} else {
+		out.children = make(children, len(n.children), cap(n.children))
+	}
+	copy(out.children, n.children)
+	return out
+}
+
+func (n *node) mutableChild(i int) *node {
+	c := n.children[i].mutableFor(n.cow)
+	n.children[i] = c
+	return c
+}
+
+// split splits the given node at the given index.  The current node shrinks,
+// and this function returns the item that existed at that index and a new node
+// containing all items/children after it.
+func (n *node) split(i int) (Item, *node) {
+	item := n.items[i]
+	next := n.cow.newNode()
+	next.items = append(next.items, n.items[i+1:]...)
+	n.items.truncate(i)
+	if len(n.children) > 0 {
+		next.children = append(next.children, n.children[i+1:]...)
+		n.children.truncate(i + 1)
+	}
+	return item, next
+}
+
+// maybeSplitChild checks if a child should be split, and if so splits it.
+// Returns whether or not a split occurred.
+func (n *node) maybeSplitChild(i, maxItems int) bool {
+	if len(n.children[i].items) < maxItems {
+		return false
+	}
+	first := n.mutableChild(i)
+	item, second := first.split(maxItems / 2)
+	n.items.insertAt(i, item)
+	n.children.insertAt(i+1, second)
+	return true
+}
+
+// insert inserts an item into the subtree rooted at this node, making sure
+// no nodes in the subtree exceed maxItems items.  Should an equivalent item be
+// be found/replaced by insert, it will be returned.
+func (n *node) insert(item Item, maxItems int) Item {
+	i, found := n.items.find(item)
+	if found {
+		out := n.items[i]
+		n.items[i] = item
+		return out
+	}
+	if len(n.children) == 0 {
+		n.items.insertAt(i, item)
+		return nil
+	}
+	if n.maybeSplitChild(i, maxItems) {
+		inTree := n.items[i]
+		switch {
+		case item.Less(inTree):
+			// no change, we want first split node
+		case inTree.Less(item):
+			i++ // we want second split node
+		default:
+			out := n.items[i]
+			n.items[i] = item
+			return out
+		}
+	}
+	return n.mutableChild(i).insert(item, maxItems)
+}
+
+// get finds the given key in the subtree and returns it.
+func (n *node) get(key Item) Item {
+	i, found := n.items.find(key)
+	if found {
+		return n.items[i]
+	} else if len(n.children) > 0 {
+		return n.children[i].get(key)
+	}
+	return nil
+}
+
+// min returns the first item in the subtree.
+func min(n *node) Item {
+	if n == nil {
+		return nil
+	}
+	for len(n.children) > 0 {
+		n = n.children[0]
+	}
+	if len(n.items) == 0 {
+		return nil
+	}
+	return n.items[0]
+}
+
+// max returns the last item in the subtree.
+func max(n *node) Item {
+	if n == nil {
+		return nil
+	}
+	for len(n.children) > 0 {
+		n = n.children[len(n.children)-1]
+	}
+	if len(n.items) == 0 {
+		return nil
+	}
+	return n.items[len(n.items)-1]
+}
+
+// toRemove details what item to remove in a node.remove call.
+type toRemove int
+
+const (
+	removeItem toRemove = iota // removes the given item
+	removeMin                  // removes smallest item in the subtree
+	removeMax                  // removes largest item in the subtree
+)
+
+// remove removes an item from the subtree rooted at this node.
+func (n *node) remove(item Item, minItems int, typ toRemove) Item {
+	var i int
+	var found bool
+	switch typ {
+	case removeMax:
+		if len(n.children) == 0 {
+			return n.items.pop()
+		}
+		i = len(n.items)
+	case removeMin:
+		if len(n.children) == 0 {
+			return n.items.removeAt(0)
+		}
+		i = 0
+	case removeItem:
+		i, found = n.items.find(item)
+		if len(n.children) == 0 {
+			if found {
+				return n.items.removeAt(i)
+			}
+			return nil
+		}
+	default:
+		panic("invalid type")
+	}
+	// If we get to here, we have children.
+	if len(n.children[i].items) <= minItems {
+		return n.growChildAndRemove(i, item, minItems, typ)
+	}
+	child := n.mutableChild(i)
+	// Either we had enough items to begin with, or we've done some
+	// merging/stealing, because we've got enough now and we're ready to return
+	// stuff.
+	if found {
+		// The item exists at index 'i', and the child we've selected can give us a
+		// predecessor, since if we've gotten here it's got > minItems items in it.
+		out := n.items[i]
+		// We use our special-case 'remove' call with typ=maxItem to pull the
+		// predecessor of item i (the rightmost leaf of our immediate left child)
+		// and set it into where we pulled the item from.
+		n.items[i] = child.remove(nil, minItems, removeMax)
+		return out
+	}
+	// Final recursive call.  Once we're here, we know that the item isn't in this
+	// node and that the child is big enough to remove from.
+	return child.remove(item, minItems, typ)
+}
+
+// growChildAndRemove grows child 'i' to make sure it's possible to remove an
+// item from it while keeping it at minItems, then calls remove to actually
+// remove it.
+//
+// Most documentation says we have to do two sets of special casing:
+//   1) item is in this node
+//   2) item is in child
+// In both cases, we need to handle the two subcases:
+//   A) node has enough values that it can spare one
+//   B) node doesn't have enough values
+// For the latter, we have to check:
+//   a) left sibling has node to spare
+//   b) right sibling has node to spare
+//   c) we must merge
+// To simplify our code here, we handle cases #1 and #2 the same:
+// If a node doesn't have enough items, we make sure it does (using a,b,c).
+// We then simply redo our remove call, and the second time (regardless of
+// whether we're in case 1 or 2), we'll have enough items and can guarantee
+// that we hit case A.
+func (n *node) growChildAndRemove(i int, item Item, minItems int, typ toRemove) Item {
+	if i > 0 && len(n.children[i-1].items) > minItems {
+		// Steal from left child
+		child := n.mutableChild(i)
+		stealFrom := n.mutableChild(i - 1)
+		stolenItem := stealFrom.items.pop()
+		child.items.insertAt(0, n.items[i-1])
+		n.items[i-1] = stolenItem
+		if len(stealFrom.children) > 0 {
+			child.children.insertAt(0, stealFrom.children.pop())
+		}
+	} else if i < len(n.items) && len(n.children[i+1].items) > minItems {
+		// steal from right child
+		child := n.mutableChild(i)
+		stealFrom := n.mutableChild(i + 1)
+		stolenItem := stealFrom.items.removeAt(0)
+		child.items = append(child.items, n.items[i])
+		n.items[i] = stolenItem
+		if len(stealFrom.children) > 0 {
+			child.children = append(child.children, stealFrom.children.removeAt(0))
+		}
+	} else {
+		if i >= len(n.items) {
+			i--
+		}
+		child := n.mutableChild(i)
+		// merge with right child
+		mergeItem := n.items.removeAt(i)
+		mergeChild := n.children.removeAt(i + 1)
+		child.items = append(child.items, mergeItem)
+		child.items = append(child.items, mergeChild.items...)
+		child.children = append(child.children, mergeChild.children...)
+		n.cow.freeNode(mergeChild)
+	}
+	return n.remove(item, minItems, typ)
+}
+
+type direction int
+
+const (
+	descend = direction(-1)
+	ascend  = direction(+1)
+)
+
+// iterate provides a simple method for iterating over elements in the tree.
+//
+// When ascending, the 'start' should be less than 'stop' and when descending,
+// the 'start' should be greater than 'stop'. Setting 'includeStart' to true
+// will force the iterator to include the first item when it equals 'start',
+// thus creating a "greaterOrEqual" or "lessThanEqual" rather than just a
+// "greaterThan" or "lessThan" queries.
+func (n *node) iterate(dir direction, start, stop Item, includeStart bool, hit bool, iter ItemIterator) (bool, bool) {
+	var ok, found bool
+	var index int
+	switch dir {
+	case ascend:
+		if start != nil {
+			index, _ = n.items.find(start)
+		}
+		for i := index; i < len(n.items); i++ {
+			if len(n.children) > 0 {
+				if hit, ok = n.children[i].iterate(dir, start, stop, includeStart, hit, iter); !ok {
+					return hit, false
+				}
+			}
+			if !includeStart && !hit && start != nil && !start.Less(n.items[i]) {
+				hit = true
+				continue
+			}
+			hit = true
+			if stop != nil && !n.items[i].Less(stop) {
+				return hit, false
+			}
+			if !iter(n.items[i]) {
+				return hit, false
+			}
+		}
+		if len(n.children) > 0 {
+			if hit, ok = n.children[len(n.children)-1].iterate(dir, start, stop, includeStart, hit, iter); !ok {
+				return hit, false
+			}
+		}
+	case descend:
+		if start != nil {
+			index, found = n.items.find(start)
+			if !found {
+				index = index - 1
+			}
+		} else {
+			index = len(n.items) - 1
+		}
+		for i := index; i >= 0; i-- {
+			if start != nil && !n.items[i].Less(start) {
+				if !includeStart || hit || start.Less(n.items[i]) {
+					continue
+				}
+			}
+			if len(n.children) > 0 {
+				if hit, ok = n.children[i+1].iterate(dir, start, stop, includeStart, hit, iter); !ok {
+					return hit, false
+				}
+			}
+			if stop != nil && !stop.Less(n.items[i]) {
+				return hit, false //	continue
+			}
+			hit = true
+			if !iter(n.items[i]) {
+				return hit, false
+			}
+		}
+		if len(n.children) > 0 {
+			if hit, ok = n.children[0].iterate(dir, start, stop, includeStart, hit, iter); !ok {
+				return hit, false
+			}
+		}
+	}
+	return hit, true
+}
+
+// Used for testing/debugging purposes.
+func (n *node) print(w io.Writer, level int) {
+	fmt.Fprintf(w, "%sNODE:%v\n", strings.Repeat("  ", level), n.items)
+	for _, c := range n.children {
+		c.print(w, level+1)
+	}
+}
+
+// BTree is an implementation of a B-Tree.
+//
+// BTree stores Item instances in an ordered structure, allowing easy insertion,
+// removal, and iteration.
+//
+// Write operations are not safe for concurrent mutation by multiple
+// goroutines, but Read operations are.
+type BTree struct {
+	degree int
+	length int
+	root   *node
+	cow    *copyOnWriteContext
+}
+
+// copyOnWriteContext pointers determine node ownership... a tree with a write
+// context equivalent to a node's write context is allowed to modify that node.
+// A tree whose write context does not match a node's is not allowed to modify
+// it, and must create a new, writable copy (IE: it's a Clone).
+//
+// When doing any write operation, we maintain the invariant that the current
+// node's context is equal to the context of the tree that requested the write.
+// We do this by, before we descend into any node, creating a copy with the
+// correct context if the contexts don't match.
+//
+// Since the node we're currently visiting on any write has the requesting
+// tree's context, that node is modifiable in place.  Children of that node may
+// not share context, but before we descend into them, we'll make a mutable
+// copy.
+type copyOnWriteContext struct {
+	freelist *FreeList
+}
+
+// Clone clones the btree, lazily.  Clone should not be called concurrently,
+// but the original tree (t) and the new tree (t2) can be used concurrently
+// once the Clone call completes.
+//
+// The internal tree structure of b is marked read-only and shared between t and
+// t2.  Writes to both t and t2 use copy-on-write logic, creating new nodes
+// whenever one of b's original nodes would have been modified.  Read operations
+// should have no performance degredation.  Write operations for both t and t2
+// will initially experience minor slow-downs caused by additional allocs and
+// copies due to the aforementioned copy-on-write logic, but should converge to
+// the original performance characteristics of the original tree.
+func (t *BTree) Clone() (t2 *BTree) {
+	// Create two entirely new copy-on-write contexts.
+	// This operation effectively creates three trees:
+	//   the original, shared nodes (old b.cow)
+	//   the new b.cow nodes
+	//   the new out.cow nodes
+	cow1, cow2 := *t.cow, *t.cow
+	out := *t
+	t.cow = &cow1
+	out.cow = &cow2
+	return &out
+}
+
+// maxItems returns the max number of items to allow per node.
+func (t *BTree) maxItems() int {
+	return t.degree*2 - 1
+}
+
+// minItems returns the min number of items to allow per node (ignored for the
+// root node).
+func (t *BTree) minItems() int {
+	return t.degree - 1
+}
+
+func (c *copyOnWriteContext) newNode() (n *node) {
+	n = c.freelist.newNode()
+	n.cow = c
+	return
+}
+
+type freeType int
+
+const (
+	ftFreelistFull freeType = iota // node was freed (available for GC, not stored in freelist)
+	ftStored                       // node was stored in the freelist for later use
+	ftNotOwned                     // node was ignored by COW, since it's owned by another one
+)
+
+// freeNode frees a node within a given COW context, if it's owned by that
+// context.  It returns what happened to the node (see freeType const
+// documentation).
+func (c *copyOnWriteContext) freeNode(n *node) freeType {
+	if n.cow == c {
+		// clear to allow GC
+		n.items.truncate(0)
+		n.children.truncate(0)
+		n.cow = nil
+		if c.freelist.freeNode(n) {
+			return ftStored
+		} else {
+			return ftFreelistFull
+		}
+	} else {
+		return ftNotOwned
+	}
+}
+
+// ReplaceOrInsert adds the given item to the tree.  If an item in the tree
+// already equals the given one, it is removed from the tree and returned.
+// Otherwise, nil is returned.
+//
+// nil cannot be added to the tree (will panic).
+func (t *BTree) ReplaceOrInsert(item Item) Item {
+	if item == nil {
+		panic("nil item being added to BTree")
+	}
+	if t.root == nil {
+		t.root = t.cow.newNode()
+		t.root.items = append(t.root.items, item)
+		t.length++
+		return nil
+	} else {
+		t.root = t.root.mutableFor(t.cow)
+		if len(t.root.items) >= t.maxItems() {
+			item2, second := t.root.split(t.maxItems() / 2)
+			oldroot := t.root
+			t.root = t.cow.newNode()
+			t.root.items = append(t.root.items, item2)
+			t.root.children = append(t.root.children, oldroot, second)
+		}
+	}
+	out := t.root.insert(item, t.maxItems())
+	if out == nil {
+		t.length++
+	}
+	return out
+}
+
+// Delete removes an item equal to the passed in item from the tree, returning
+// it.  If no such item exists, returns nil.
+func (t *BTree) Delete(item Item) Item {
+	return t.deleteItem(item, removeItem)
+}
+
+// DeleteMin removes the smallest item in the tree and returns it.
+// If no such item exists, returns nil.
+func (t *BTree) DeleteMin() Item {
+	return t.deleteItem(nil, removeMin)
+}
+
+// DeleteMax removes the largest item in the tree and returns it.
+// If no such item exists, returns nil.
+func (t *BTree) DeleteMax() Item {
+	return t.deleteItem(nil, removeMax)
+}
+
+func (t *BTree) deleteItem(item Item, typ toRemove) Item {
+	if t.root == nil || len(t.root.items) == 0 {
+		return nil
+	}
+	t.root = t.root.mutableFor(t.cow)
+	out := t.root.remove(item, t.minItems(), typ)
+	if len(t.root.items) == 0 && len(t.root.children) > 0 {
+		oldroot := t.root
+		t.root = t.root.children[0]
+		t.cow.freeNode(oldroot)
+	}
+	if out != nil {
+		t.length--
+	}
+	return out
+}
+
+// AscendRange calls the iterator for every value in the tree within the range
+// [greaterOrEqual, lessThan), until iterator returns false.
+func (t *BTree) AscendRange(greaterOrEqual, lessThan Item, iterator ItemIterator) {
+	if t.root == nil {
+		return
+	}
+	t.root.iterate(ascend, greaterOrEqual, lessThan, true, false, iterator)
+}
+
+// AscendLessThan calls the iterator for every value in the tree within the range
+// [first, pivot), until iterator returns false.
+func (t *BTree) AscendLessThan(pivot Item, iterator ItemIterator) {
+	if t.root == nil {
+		return
+	}
+	t.root.iterate(ascend, nil, pivot, false, false, iterator)
+}
+
+// AscendGreaterOrEqual calls the iterator for every value in the tree within
+// the range [pivot, last], until iterator returns false.
+func (t *BTree) AscendGreaterOrEqual(pivot Item, iterator ItemIterator) {
+	if t.root == nil {
+		return
+	}
+	t.root.iterate(ascend, pivot, nil, true, false, iterator)
+}
+
+// Ascend calls the iterator for every value in the tree within the range
+// [first, last], until iterator returns false.
+func (t *BTree) Ascend(iterator ItemIterator) {
+	if t.root == nil {
+		return
+	}
+	t.root.iterate(ascend, nil, nil, false, false, iterator)
+}
+
+// DescendRange calls the iterator for every value in the tree within the range
+// [lessOrEqual, greaterThan), until iterator returns false.
+func (t *BTree) DescendRange(lessOrEqual, greaterThan Item, iterator ItemIterator) {
+	if t.root == nil {
+		return
+	}
+	t.root.iterate(descend, lessOrEqual, greaterThan, true, false, iterator)
+}
+
+// DescendLessOrEqual calls the iterator for every value in the tree within the range
+// [pivot, first], until iterator returns false.
+func (t *BTree) DescendLessOrEqual(pivot Item, iterator ItemIterator) {
+	if t.root == nil {
+		return
+	}
+	t.root.iterate(descend, pivot, nil, true, false, iterator)
+}
+
+// DescendGreaterThan calls the iterator for every value in the tree within
+// the range (pivot, last], until iterator returns false.
+func (t *BTree) DescendGreaterThan(pivot Item, iterator ItemIterator) {
+	if t.root == nil {
+		return
+	}
+	t.root.iterate(descend, nil, pivot, false, false, iterator)
+}
+
+// Descend calls the iterator for every value in the tree within the range
+// [last, first], until iterator returns false.
+func (t *BTree) Descend(iterator ItemIterator) {
+	if t.root == nil {
+		return
+	}
+	t.root.iterate(descend, nil, nil, false, false, iterator)
+}
+
+// Get looks for the key item in the tree, returning it.  It returns nil if
+// unable to find that item.
+func (t *BTree) Get(key Item) Item {
+	if t.root == nil {
+		return nil
+	}
+	return t.root.get(key)
+}
+
+// Min returns the smallest item in the tree, or nil if the tree is empty.
+func (t *BTree) Min() Item {
+	return min(t.root)
+}
+
+// Max returns the largest item in the tree, or nil if the tree is empty.
+func (t *BTree) Max() Item {
+	return max(t.root)
+}
+
+// Has returns true if the given key is in the tree.
+func (t *BTree) Has(key Item) bool {
+	return t.Get(key) != nil
+}
+
+// Len returns the number of items currently in the tree.
+func (t *BTree) Len() int {
+	return t.length
+}
+
+// Clear removes all items from the btree.  If addNodesToFreelist is true,
+// t's nodes are added to its freelist as part of this call, until the freelist
+// is full.  Otherwise, the root node is simply dereferenced and the subtree
+// left to Go's normal GC processes.
+//
+// This can be much faster
+// than calling Delete on all elements, because that requires finding/removing
+// each element in the tree and updating the tree accordingly.  It also is
+// somewhat faster than creating a new tree to replace the old one, because
+// nodes from the old tree are reclaimed into the freelist for use by the new
+// one, instead of being lost to the garbage collector.
+//
+// This call takes:
+//   O(1): when addNodesToFreelist is false, this is a single operation.
+//   O(1): when the freelist is already full, it breaks out immediately
+//   O(freelist size):  when the freelist is empty and the nodes are all owned
+//       by this tree, nodes are added to the freelist until full.
+//   O(tree size):  when all nodes are owned by another tree, all nodes are
+//       iterated over looking for nodes to add to the freelist, and due to
+//       ownership, none are.
+func (t *BTree) Clear(addNodesToFreelist bool) {
+	if t.root != nil && addNodesToFreelist {
+		t.root.reset(t.cow)
+	}
+	t.root, t.length = nil, 0
+}
+
+// reset returns a subtree to the freelist.  It breaks out immediately if the
+// freelist is full, since the only benefit of iterating is to fill that
+// freelist up.  Returns true if parent reset call should continue.
+func (n *node) reset(c *copyOnWriteContext) bool {
+	for _, child := range n.children {
+		if !child.reset(c) {
+			return false
+		}
+	}
+	return c.freeNode(n) != ftFreelistFull
+}
+
+// Int implements the Item interface for integers.
+type Int int
+
+// Less returns true if int(a) < int(b).
+func (a Int) Less(b Item) bool {
+	return a < b.(Int)
+}