code: @Override
    public ImmutableSetMultimap<V, K> inverse() {
      ImmutableSetMultimap<V, K> result = inverse;
      return (result == null) ? (inverse = invert()) : result;
    }
docs: {@inheritDoc}
  
  <p>Because an inverse of a set multimap cannot contain multiple pairs with the same key and
  value, this method returns an {@code ImmutableSetMultimap} rather than the {@code
  ImmutableMultimap} specified in the {@code ImmutableMultimap} class.