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