android/guava/src/com/google/common/collect/ImmutableSetMultimap.java
@Override
  public ImmutableSetMultimap<V, K> inverse() {
    ImmutableSetMultimap<V, K> result = inverse;
    return (result == null) ? (inverse = invert()) : result;
  }
@Override
  public ImmutableSetMultimap<V, K> inverse() {
    ImmutableSetMultimap<V, K> result = inverse;
    return (result == null) ? (inverse = invert()) : result;
  }
{@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.