theorem Th125:
  product <*D1,D2*> = the set of all <*d1,d2*>
proof
  thus product <*D1,D2*> c= the set of all <*d1,d2*>
  proof
    let a be object;
    assume a in product <*D1,D2*>;
    then ex x,y st x in D1 & y in D2 & a = <*x,y*> by Th122;
    hence thesis;
  end;
  let a be object;
  assume a in the set of all <*d1,d2*>;
  then ex d1,d2 st a = <*d1,d2*>;
  hence thesis by Th122;
end;
