theorem
  for f being Morphism of C st cod(f) = b holds (id b)(*)f = f
   proof let f be Morphism of C;
    assume
A1:   cod f = b;
     then reconsider ff=f as Morphism of dom f,b by Th3;
      Hom(dom f,b)<>{} by A1,Th1;
     then (id b)(*)ff = ff by Def10;
    hence thesis;
   end;
