code: public static <K, V> ImmutableMap<K, V> of(K k1, V v1) {
      return ImmutableBiMap.of(k1, v1);
    }
docs: Returns an immutable map containing a single entry. This map behaves and performs comparably to
  {@link Collections#singletonMap} but will not accept a null key or value. It is preferable
  mainly for consistency and maintainability of your code.