reserve UA for Universal_Algebra,
  f, g for Function of UA, UA;

theorem Th6:
  for f1, f2 be Element of UAAut UA holds f1 * f2 in UAAut UA
proof
  let f1, f2 be Element of UAAut UA;
  f1 is_isomorphism & f2 is_isomorphism by Def1;
  then f1 * f2 is_isomorphism by ALG_1:11;
  hence thesis by Def1;
end;
