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diff --git a/xdelta3/cpp-btree/safe_btree_set.h b/xdelta3/cpp-btree/safe_btree_set.h
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+// Copyright 2013 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.
+//
+// The safe_btree_set<> is like btree_set<> except that it removes the caveat
+// about insertion and deletion invalidating existing iterators at a small cost
+// in making iterators larger and slower.
+//
+// Revalidation occurs whenever an iterator is accessed.  References
+// and pointers returned by safe_btree_map<> iterators are not stable,
+// they are potentially invalidated by any non-const method on the set.
+//
+// BEGIN INCORRECT EXAMPLE
+//   for (auto i = safe_set->begin(); i != safe_set->end(); ++i) {
+//     const T &value = *i;  // DO NOT DO THIS
+//     [code that modifies safe_set and uses value];
+//   }
+// END INCORRECT EXAMPLE
+
+#ifndef UTIL_BTREE_SAFE_BTREE_SET_H__
+#define UTIL_BTREE_SAFE_BTREE_SET_H__
+
+#include <functional>
+#include <memory>
+
+#include "btree_container.h"
+#include "btree_set.h"
+#include "safe_btree.h"
+
+namespace btree {
+
+// The safe_btree_set class is needed mainly for its constructors.
+template <typename Key,
+          typename Compare = std::less<Key>,
+          typename Alloc = std::allocator<Key>,
+          int TargetNodeSize = 256>
+class safe_btree_set : public btree_unique_container<
+  safe_btree<btree_set_params<Key, Compare, Alloc, TargetNodeSize> > > {
+
+  typedef safe_btree_set<Key, Compare, Alloc, TargetNodeSize> self_type;
+  typedef btree_set_params<Key, Compare, Alloc, TargetNodeSize> params_type;
+  typedef safe_btree<params_type> btree_type;
+  typedef btree_unique_container<btree_type> super_type;
+
+ public:
+  typedef typename btree_type::key_compare key_compare;
+  typedef typename btree_type::allocator_type allocator_type;
+
+ public:
+  // Default constructor.
+  safe_btree_set(const key_compare &comp = key_compare(),
+                 const allocator_type &alloc = allocator_type())
+      : super_type(comp, alloc) {
+  }
+
+  // Copy constructor.
+  safe_btree_set(const self_type &x)
+      : super_type(x) {
+  }
+
+  // Range constructor.
+  template <class InputIterator>
+  safe_btree_set(InputIterator b, InputIterator e,
+                 const key_compare &comp = key_compare(),
+                 const allocator_type &alloc = allocator_type())
+      : super_type(b, e, comp, alloc) {
+  }
+};
+
+template <typename K, typename C, typename A, int N>
+inline void swap(safe_btree_set<K, C, A, N> &x,
+                 safe_btree_set<K, C, A, N> &y) {
+  x.swap(y);
+}
+
+} // namespace btree
+
+#endif  // UTIL_BTREE_SAFE_BTREE_SET_H__