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

theorem
  for FT being filled non empty RelStr, A being Subset of FT holds A
  ^deltao=(A^b) \ A
proof
  let FT be filled non empty RelStr,A be Subset of FT;
A1: A` /\ ((A`)^b)=A` by FIN_TOPO:13,XBOOLE_1:28;
  thus A^deltao=A` /\ ((A^b) /\ ((A`)^b)) by FIN_TOPO:18
    .= (A^b) /\ (A` /\ ((A`)^b)) by XBOOLE_1:16
    .= A^b \ A by A1,SUBSET_1:13;
end;
