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diff --git a/guava/src/com/google/common/collect/BstModifier.java b/guava/src/com/google/common/collect/BstModifier.java
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+++ b/guava/src/com/google/common/collect/BstModifier.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 specification for a local change to an entry in a binary search tree.
+ *
+ * @author Louis Wasserman
+ */
+@GwtCompatible
+interface BstModifier<K, N extends BstNode<K, N>> {
+
+  /**
+   * Given a target key and the original entry (if any) with the specified key, returns the entry
+   * with key {@code key} after this mutation has been performed. The result must either be {@code
+   * null} or must have a key that compares as equal to {@code key}. A deletion operation, for
+   * example, would always return {@code null}, or an insertion operation would always return a
+   * non-null {@code insertedEntry}.
+   *
+   * <p>If this method returns a non-null entry of type {@code N}, any children it has will be
+   * ignored.
+   *
+   * <p>This method may return {@code originalEntry} itself to indicate that no change is made.
+   *
+   * @param key The key being targeted for modification.
+   * @param originalEntry The original entry in the binary search tree with the specified key, if
+   *        any. No guarantees are made about the children of this entry when treated as a node; in
+   *        particular, they are not necessarily the children of the corresponding node in the
+   *        binary search tree.
+   * @return the entry (if any) with the specified key after this modification is performed
+   */
+  BstModificationResult<N> modify(K key, @Nullable N originalEntry);
+}