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diff --git a/src/main/java/org/whispersystems/libaxolotl/util/guava/Function.java b/src/main/java/org/whispersystems/libaxolotl/util/guava/Function.java
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+/*
+ * Copyright (C) 2007 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 org.whispersystems.libaxolotl.util.guava;
+
+
+
+/**
+ * Determines an output value based on an input value.
+ * 
+ * <p>See the Guava User Guide article on <a href=
+ * "http://code.google.com/p/guava-libraries/wiki/FunctionalExplained">the use of {@code
+ * Function}</a>.
+ * 
+ * @author Kevin Bourrillion
+ * @since 2.0 (imported from Google Collections Library)
+ */
+
+public interface Function<F, T> {
+  /**
+   * Returns the result of applying this function to {@code input}. This method is <i>generally
+   * expected</i>, but not absolutely required, to have the following properties:
+   *
+   * <ul>
+   * <li>Its execution does not cause any observable side effects.
+   * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal
+   *     Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a),
+   *     function.apply(b))}.
+   * </ul>
+   *
+   * @throws NullPointerException if {@code input} is null and this function does not accept null
+   *     arguments
+   */
+  T apply(F input);
+
+  /**
+   * Indicates whether another object is equal to this function.
+   *
+   * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
+   * However, an implementation may also choose to return {@code true} whenever {@code object} is a
+   * {@link Function} that it considers <i>interchangeable</i> with this one. "Interchangeable"
+   * <i>typically</i> means that {@code Objects.equal(this.apply(f), that.apply(f))} is true for all
+   * {@code f} of type {@code F}. Note that a {@code false} result from this method does not imply
+   * that the functions are known <i>not</i> to be interchangeable.
+   */
+  @Override
+  boolean equals(Object object);
+}