code: func (p *path) pop() *pathBundle {
  	if pb := p.tail; pb != nil {
  		p.tail = pb.prev
  		pb.prev = nil
  		return pb
  	}
  
  	return nil
  }
docs: // pop removes the last item from the path.  Note that it also nils
  // out the returned pathBundle's prev field.  Returns nil if no items
  // remain.