
theorem Th1:
  for X being set, b1,b2 being bag of X holds (b1 + b2) / b2 = b1
proof
  let X be set, b1,b2 be bag of X;
  b2 divides b1 + b2 by PRE_POLY:50;
  then b2 + ((b1 + b2) / b2) = b1 + b2 by GROEB_2:def 1;
  then (b2 + ((b1 + b2) / b2)) -' b2 = b1 by PRE_POLY:48;
  hence thesis by PRE_POLY:48;
end;
