* There is no need to use this method on a {@link Map} implementation that already has a
   * corresponding {@link Set} implementation (such as {@link java.util.HashMap} or {@link
   * java.util.TreeMap}).
   *
   * <p>Each method invocation on the set returned by this method results in exactly one method
   * invocation on the backing map or its {@code keySet} view, with one exception. The {@code