reserve i for Nat,
  j for Element of NAT,
  X,Y,x,y,z for set;

theorem Th4:
  (id X).:Y c= Y
proof
  let x be object;
  assume x in (id X).:Y;
  then ex y being object st [y,x] in id X & y in Y by RELAT_1:def 13;
  hence thesis by RELAT_1:def 10;
end;
