func (s *Iterator) SeekForPrev(target []byte) {
	s.n, _ = s.list.findNear(target, true, true) // find <=.
}
func (s *Iterator) SeekForPrev(target []byte) {
	s.n, _ = s.list.findNear(target, true, true) // find <=.
}
// SeekForPrev finds an entry with key <= target.