theorem ThB33:
  A * B c= (-B) + A + B
proof
  let x be object;
  assume x in A * B;
  then consider a,b such that
A1: x = a * b and
A2: a in A and
A3: b in B;
  (-b) in (-B) by A3;
  then (-b) + a in (-B) + A by A2;
  hence thesis by A1,A3;
end;
