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diff --git a/guava/src/com/google/common/collect/BstNodeFactory.java b/guava/src/com/google/common/collect/BstNodeFactory.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 factory for copying nodes in binary search trees with different children.
+ *
+ * <p>Typically, nodes will carry more information than the fields in the {@link BstNode} class,
+ * often some kind of value or some aggregate data for the subtree. This factory is responsible for
+ * copying this additional data between nodes.
+ *
+ * @author Louis Wasserman
+ * @param <N> The type of the tree nodes constructed with this {@code BstNodeFactory}.
+ */
+@GwtCompatible
+abstract class BstNodeFactory<N extends BstNode<?, N>> {
+  /**
+   * Returns a new {@code N} with the key and value data from {@code source}, with left child
+   * {@code left}, and right child {@code right}. If {@code left} or {@code right} is null, the
+   * returned node will not have a child on the corresponding side.
+   */
+  public abstract N createNode(N source, @Nullable N left, @Nullable N right);
+
+  /**
+   * Returns a new {@code N} with the key and value data from {@code source} that is a leaf.
+   */
+  public final N createLeaf(N source) {
+    return createNode(source, null, null);
+  }
+}