func (s *MergeIterator) Rewind() {
	for _, itr := range s.all {
		itr.Rewind()
	}
	s.initHeap()
}
func (s *MergeIterator) Rewind() {
	for _, itr := range s.all {
		itr.Rewind()
	}
	s.initHeap()
}
// Rewind seeks to first element (or last element for reverse iterator).