class ConcurrentHashMultimap implements Multimap<K, V> {

    @Override
    public Multiset<K> keys() {
        final ImmutableMultiset.Builder<K> builder = ImmutableMultiset.builder();
        entries.forEach((key, values) -> builder.setCount(key, values.size()));
        return builder.build();
    }

    public  ConcurrentHashMultimap(ValueIdentityExtractor<V> valueIdentityExtractor, ConflictResolver<V> conflictResolver);

}