@Override
        public boolean retainAll(Collection<?> c) {
          return delegate.entrySet().retainAll(c);
        }
      };
    }
  }

  @Override
  protected Map<String, String> makeEmptyMap() {
    Map<String, Integer> underlying = new HashMap<>();
    return transformValues(
        new UnmodifiableIteratorMap<String, Integer>(underlying), Functions.toStringFunction());
  }

  @Override