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diff --git a/java/com/google/gerrit/server/rules/StoredValue.java b/java/com/google/gerrit/server/rules/StoredValue.java
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+++ b/java/com/google/gerrit/server/rules/StoredValue.java
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+// Copyright (C) 2011 The Android Open Source Project
+//
+// 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.gerrit.server.rules;
+
+import com.googlecode.prolog_cafe.exceptions.SystemException;
+import com.googlecode.prolog_cafe.lang.Prolog;
+
+/**
+ * Defines a value cached in a {@link PrologEnvironment}.
+ *
+ * @see StoredValues
+ */
+public class StoredValue<T> {
+  /** Construct a new unique key that does not match any other key. */
+  public static <T> StoredValue<T> create() {
+    return new StoredValue<>();
+  }
+
+  /** Construct a key based on a Java Class object, useful for singletons. */
+  public static <T> StoredValue<T> create(Class<T> clazz) {
+    return new StoredValue<>(clazz);
+  }
+
+  private final Object key;
+
+  /**
+   * Initialize a stored value key using any Java Object.
+   *
+   * @param key unique identity of the stored value. This will be the hash key in the Prolog
+   *     Environments's hash map.
+   */
+  public StoredValue(Object key) {
+    this.key = key;
+  }
+
+  /** Initializes a stored value key with a new unique key. */
+  public StoredValue() {
+    key = this;
+  }
+
+  /** Look up the value in the engine, or return null. */
+  public T getOrNull(Prolog engine) {
+    return get((PrologEnvironment) engine.control);
+  }
+  /** Get the value from the engine, or throw SystemException. */
+  public T get(Prolog engine) {
+    T obj = getOrNull(engine);
+    if (obj == null) {
+      // unless createValue() is overridden, will return null
+      obj = createValue(engine);
+      if (obj == null) {
+        throw new SystemException("No " + key + " available");
+      }
+      set(engine, obj);
+    }
+    return obj;
+  }
+
+  public void set(Prolog engine, T obj) {
+    set((PrologEnvironment) engine.control, obj);
+  }
+
+  /** Perform {@link #getOrNull(Prolog)} on the environment's interpreter. */
+  public T get(PrologEnvironment env) {
+    return env.get(this);
+  }
+
+  /** Set the value into the environment's interpreter. */
+  public void set(PrologEnvironment env, T obj) {
+    env.set(this, obj);
+  }
+
+  /**
+   * Creates a value to store, returns null by default.
+   *
+   * @param engine Prolog engine.
+   * @return new value.
+   */
+  protected T createValue(Prolog engine) {
+    return null;
+  }
+}