reserve FT for non empty RelStr;
reserve x, y, z for Element of FT;
reserve A for Subset of FT;
reserve F for Subset of FT;

theorem Th13:
  for FT being filled non empty RelStr, A being Subset of FT holds A c= A^b
proof
  let FT be filled non empty RelStr;
  let A be Subset of FT;
  let x be object;
  assume
A1: x in A;
  then reconsider y=x as Element of FT;
  y in U_FT y by Def4;
  then U_FT y meets A by A1,XBOOLE_0:3;
  hence thesis;
end;
