code: @SuppressWarnings("ConstantConditions")
    private ImmutableBiMap<K,V> toImmutableBiMap() {
      final ImmutableBiMap.Builder<K,V> ret = ImmutableBiMap.builder();
  
      for (final MapEntry<K, V> entry : entries) {
        ret.put(entry.key, entry.value);
      }
  
      return ret.build();
    }
docs: if any element is null