reserve a,b for object, I,J for set;
reserve b for bag of I;

theorem Th23:
  Sum <*b*> = b
  proof
    thus Sum <*b*> = Sum ((<*>Bags I)^<*b*>) by FINSEQ_1:34
    .= Sum <*>Bags I + b by Th22
    .= EmptyBag I + b by Th21
    .= b by PRE_POLY:53;
  end;
