theorem Th3:
  for F being strict RingMorphism holds F is Morphism of dom(F),cod
  (F) & dom(F) <= cod(F)
proof
  let F be strict RingMorphism;
  ex G,H st G <= H & dom(F) = G & cod(F) = H & F is Morphism of G,H by Lm6;
  hence thesis;
end;
