builder.put(keyFunction.apply(value), value);
    }
    return builder.build();
  }

  static class Keys<K extends @Nullable Object, V extends @Nullable Object>
      extends AbstractMultiset<K> {
    @Weak final Multimap<K, V> multimap;

    Keys(Multimap<K, V> multimap) {
      this.multimap = multimap;
    }

    @Override
    Iterator<Multiset.Entry<K>> entryIterator() {