
theorem Th12:
  for R, S being non empty RelStr for x being set st x in (the
  carrier of R) /\ (the carrier of S) holds x is Element of R
proof
  let R, S be non empty RelStr;
  let x be set;
  assume
A1: x in (the carrier of R) /\ (the carrier of S);
  (the carrier of R) /\ (the carrier of S) c= the carrier of R by XBOOLE_1:17;
  hence thesis by A1;
end;
