code: func (s *parentStack) At(n int) *treapNode {
  	index := s.index - n - 1
  	if index < 0 {
  		return nil
  	}
  
  	if index < staticDepth {
  		return s.items[index]
  	}
  
  	return s.overflow[index-staticDepth]
  }
docs: // At returns the item n number of items from the top of the stack, where 0 is
  // the topmost item, without removing it.  It returns nil if n exceeds the
  // number of items on the stack.