guava/src/com/google/common/cache/LocalCache.java
@SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {
    return (ValueReference<K, V>) UNSET;
  }
@SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {
    return (ValueReference<K, V>) UNSET;
  }
Singleton placeholder that indicates a value is being loaded.