theorem
  for M1, M2 being set st card M1 = 0 & card M2 = 0 holds M1 = M2
proof
  let M1, M2 be set;
  assume that
A1: card M1 = 0 and
A2: card M2 = 0;
  M1 = {} by A1;
  hence thesis by A2;
end;
