code: func (p *PopOp) CanPop(id string) bool {
  	if _, hasTomb := p.tombs[id]; hasTomb {
  		return false // already popped by someone else
  	}
  	if _, present := p.idToKeys[id]; present {
  		return true // listed in the set
  	}
  	return false
  }
docs: // CanPop returns true if the given item can be popped from the set.
  //
  // Returns false if this item has been popped before (perhaps in another
  // transaction), or it's not in the the listing passed to BeginPop.