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diff --git a/guava/src/com/google/common/collect/BstBalancePolicy.java b/guava/src/com/google/common/collect/BstBalancePolicy.java
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+++ b/guava/src/com/google/common/collect/BstBalancePolicy.java
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+/*
+ * Copyright (C) 2011 The Guava 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 com.google.common.collect;
+
+import com.google.common.annotations.GwtCompatible;
+
+import javax.annotation.Nullable;
+
+/**
+ * A local balancing policy for modified nodes in binary search trees.
+ *
+ * @author Louis Wasserman
+ * @param <N> The type of the nodes in the trees that this {@code BstRebalancePolicy} can
+ *        rebalance.
+ */
+@GwtCompatible
+interface BstBalancePolicy<N extends BstNode<?, N>> {
+  /**
+   * Constructs a locally balanced tree around the key and value data in {@code source}, and the
+   * subtrees {@code left} and {@code right}. It is guaranteed that the resulting tree will have
+   * the same inorder traversal order as the subtree {@code left}, then the entry {@code source},
+   * then the subtree {@code right}.
+   */
+  N balance(BstNodeFactory<N> nodeFactory, N source, @Nullable N left, @Nullable N right);
+
+  /**
+   * Constructs a locally balanced tree around the subtrees {@code left} and {@code right}. It is
+   * guaranteed that the resulting tree will have the same inorder traversal order as the subtree
+   * {@code left}, then the subtree {@code right}.
+   */
+  @Nullable
+  N combine(BstNodeFactory<N> nodeFactory, @Nullable N left, @Nullable N right);
+}