code: func (p *PopOp) Pop(id string) bool {
  	if p.finished {
  		panic("the operation has already been finished")
  	}
  	if !p.CanPop(id) {
  		return false
  	}
  	p.tombs[id] = p.now
  	p.popped = append(p.popped, &tombstone{
  		id:      id,
  		storage: p.idToKeys[id],
  	})
  	p.dirty = true
  	return true
  }
docs: // Pop removed the item from the set and returns true if it was there.
  //
  // Returns false if this item has been popped before (perhaps in another
  // transaction), or it's not in the the listing passed to BeginPop.