
theorem
  for S, T being up-complete non empty Poset for f being Function of S
  , T holds f is isomorphic iff Sigma f is isomorphic
proof
  let S, T be up-complete non empty Poset;
  let f be Function of S, T;
  the RelStr of Sigma S = the RelStr of S & the RelStr of Sigma T = the
  RelStr of T by YELLOW_9:def 4;
  hence thesis by Th5;
end;
