if (entry instanceof ImmutableEntry) {
        checkNotNull(entry.getKey());
        checkNotNull(entry.getValue());
        @SuppressWarnings("unchecked") // all supported methods are covariant
        Entry<K, V> immutableEntry = (Entry<K, V>) entry;
        entries.add(immutableEntry);
      } else {
        entries.add(entryOf((K) entry.getKey(), (V) entry.getValue()));
      }
      return this;
    }

          final Entry<K, V> backingEntry = backingIterator.next();
          return new AbstractMapEntry<K, ImmutableSet<V>>() {
            @Override
            public K getKey() {
              return backingEntry.getKey();
            }

            @Override
            public ImmutableSet<V> getValue() {
              return ImmutableSet.of(backingEntry.getValue());
            }
          };

        return entryIterator.hasNext();
      }

      @Override
      public K next() {
        return entryIterator.next().getKey();
      }
    };
  }

  Spliterator<K> keySpliterator() {
    return CollectSpliterators.map(entrySet().spliterator(), Entry::getKey);
  }

  @LazyInit @RetainedWith @CheckForNull private transient ImmutableCollection<V> values;

  /**