
theorem Th3:
  for X, a, b being set, f being Function of X,{a,b} holds X = f"{a } \/ f"{b}
proof
  let X, a, b be set;
  let f be Function of X,{a,b};
  thus X = f"{a,b} by FUNCT_2:40
    .= f"({a}\/{b}) by ENUMSET1:1
    .= f"{a} \/ f"{b} by RELAT_1:140;
end;
