func (s *Skiplist) MakeBuf() *ActionBuffer {
	return &ActionBuffer{
		preds: make([]*Node, MaxLevel+1),
		succs: make([]*Node, MaxLevel+1),
	}
}
func (s *Skiplist) MakeBuf() *ActionBuffer {
	return &ActionBuffer{
		preds: make([]*Node, MaxLevel+1),
		succs: make([]*Node, MaxLevel+1),
	}
}
// MakeBuf creates an action buffer