theorem
  for W being Universe, a,b being Object of Ens(W) st Hom(a,b) <> {}
  for f being Morphism of a,b st f is epi holds @f
  is surjective
proof
  let W be Universe, a,b being Object of Ens(W) such that
 Hom(a,b) <> {};
  let f be Morphism of a,b;
  {} in W & {{}} in W by CLASSES2:56,57;
  then
A1: {{},{{}}} in W by CLASSES2:58;
  {} in {{},{{}}} & {{}} in {{},{{}}} by TARSKI:def 2;
  hence thesis by A1,Th36;
end;
