reserve C for CatStr;
reserve f,g for Morphism of C;
reserve C for non void non empty CatStr,
  f,g for Morphism of C,
  a,b,c,d for Object of C;

theorem Th3:
  for f being Morphism of C holds f is Morphism of dom(f),cod(f)
proof
  let f be Morphism of C;
  f in Hom(dom(f),cod(f));
  hence thesis by Def3;
end;
