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diff --git a/guava/src/com/google/common/collect/BstPathFactory.java b/guava/src/com/google/common/collect/BstPathFactory.java
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+++ b/guava/src/com/google/common/collect/BstPathFactory.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;
+
+/**
+ * A factory for extending paths in a binary search tree.
+ *
+ * @author Louis Wasserman
+ * @param <N> The type of binary search tree nodes used in the paths generated by this {@code
+ *        BstPathFactory}.
+ * @param <P> The type of paths constructed by this {@code BstPathFactory}.
+ */
+@GwtCompatible
+interface BstPathFactory<N extends BstNode<?, N>, P extends BstPath<N, P>> {
+  /**
+   * Returns this path extended by one node to the specified {@code side}.
+   */
+  P extension(P path, BstSide side);
+
+  /**
+   * Returns the trivial path that starts at {@code root} and goes no further.
+   */
+  P initialPath(N root);
+}