reserve FT for non empty RelStr,
  A,B,C for Subset of FT;

theorem Th9:
  for X9 being SubSpace of FT, A being Subset of X9 holds A is Subset of FT
proof
  let X9 be SubSpace of FT, A be Subset of X9;
  the carrier of X9 c= the carrier of FT by Def2;
  hence thesis by XBOOLE_1:1;
end;
