theorem
  for A,B being non empty set, f being Function of A,B st for x being
  Element of A holds f.x in Z holds rng f c= Z
proof
  let A,B be non empty set, f be Function of A,B such that
A1: for x being Element of A holds f.x in Z;
  let y be object;
  assume y in rng f;
  then ex x being Element of A st f.x=y by Th112;
  hence thesis by A1;
end;
