* read operations will work correctly if the last write <i>happens-before</i> any reads. To allow
 * concurrent update operations, wrap your multimap with a call to {@link
 * Multimaps#synchronizedSetMultimap}.
 *
 * <p><b>Warning:</b> Do not modify either a key <i>or a value</i> of a {@code HashMultimap} in a
 * way that affects its {@link Object#equals} behavior. Undefined behavior and bugs will result.
 *