reserve x,y for set;
reserve D for non empty set;
reserve UN for Universe;
reserve f for RingMorphismStr;
reserve G,H,G1,G2,G3,G4 for Ring;
reserve F for RingMorphism;
reserve V for Ring_DOMAIN;

theorem Th12:
  for f being strict RingMorphism holds {f} is RingMorphism_DOMAIN
proof
  let f be strict RingMorphism;
  for x being object st x in {f} holds x is strict RingMorphism
    by TARSKI:def 1;
  hence thesis by Def12;
end;
