
theorem Th3:
  for L be RelStr for S be SubRelStr of L for X be Subset of S
  holds X is Subset of L
proof
  let L be RelStr;
  let S be SubRelStr of L;
  let X be Subset of S;
  the carrier of S c= the carrier of L by YELLOW_0:def 13;
  hence thesis by XBOOLE_1:1;
end;
