*
   * <p>Changes in the underlying multimap are reflected in this view. Conversely, this view
   * supports removal operations, and these are reflected in the underlying multimap.
   *
   * <p>It's acceptable for the underlying multimap to contain null keys, and even null values
   * provided that the function is capable of accepting null input. The transformed multimap might

   *
   * <p>Changes in the underlying multimap are reflected in this view. Conversely, this view
   * supports removal operations, and these are reflected in the underlying multimap.
   *
   * <p>It's acceptable for the underlying multimap to contain null keys, and even null values
   * provided that the function is capable of accepting null input. The transformed multimap might

     * IllegalArgumentException, since the call could succeed with a different argument. Those
     * exceptions' docs suggest that either is acceptable. Google's Java Practices page recommends
     * IllegalArgumentException here, in part to keep its recommendation simple: Static methods
     * should throw IllegalStateException only when they use static state.
     *