reserve FT for non empty RelStr;
reserve A for Subset of FT;

theorem
  A^delta = A^b \ A^i
proof
  for x being object holds x in A^delta iff x in A^b \ A^i
  proof
    let x be object;
    thus x in A^delta implies x in A^b \ A^i
    proof
      assume x in A^delta;
      then x in ((A^b) /\ ((A^i)`)) by Th2;
      hence thesis by SUBSET_1:13;
    end;
    assume x in A^b \ A^i;
    then x in ((A^b) /\ ((A^i)`)) by SUBSET_1:13;
    hence thesis by Th2;
  end;
  hence thesis by TARSKI:2;
end;
