Multiset.Entry<K> transform(final Map.Entry<K, Collection<V>> backingEntry) {
          return new Multisets.AbstractEntry<K>() {
            @Override
            @ParametricNullness
            public K getElement() {
              return backingEntry.getKey();
            }

            @Override
            public int getCount() {
              return backingEntry.getValue().size();
            }