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

theorem
  for A being Subset of FT holds A^deltai misses A^deltao
proof
  let A be Subset of FT;
  A misses A` by XBOOLE_1:79;
  then
A1: A /\ A` = {};
  thus A^deltai /\ (A^deltao) = A /\ ((A^delta) /\ A`) /\ (A^delta) by
XBOOLE_1:16
    .= A /\ A` /\ (A^delta) /\ (A^delta) by XBOOLE_1:16
    .= {} by A1;
end;
