theorem
  for A,B being Subset of QuasiAdjs C st A c= B holds A at f c= B at f
proof
  let A,B be Subset of QuasiAdjs C;
  assume A c= B;
  then A\/B = B by XBOOLE_1:12;
  then B at f = (A at f)\/(B at f) by Th145;
  hence thesis by XBOOLE_1:7;
end;
