public boolean contains(@CheckForNull Object element) {
      return multimap.containsKey(element);
    }

    @Override
    public Iterator<K> iterator() {
      return Maps.keyIterator(multimap.entries().iterator());
    }

    @Override
    public int count(@CheckForNull Object element) {
      Collection<V> values = Maps.safeGet(multimap.asMap(), element);
      return (values == null) ? 0 : values.size();