
theorem
  for b being bag of {} holds b = EmptyBag {}
proof
  set n = {};
  let b be bag of {};
A1: for b being bag of n holds b = {}
  proof
    let b be bag of n;
    b in Bags n by PRE_POLY:def 12;
    hence thesis by PRE_POLY:51,TARSKI:def 1;
  end;
  then EmptyBag n = {};
  hence thesis by A1;
end;
