code: func (it *Optional) Contains(ctx context.Context, val graph.Value) bool {
  	checked := it.subIt.Contains(ctx, val)
  	it.lastCheck = checked
  	it.err = it.subIt.Err()
  	it.result = val
  	return true
  }
docs: // Contains() is the real hack of this iterator. It always returns true, regardless
  // of whether the subiterator matched. But we keep track of whether the subiterator
  // matched for results purposes.