reserve A,B,C,D for Category,
  F for Functor of A,B,
  G for Functor of B,C;

theorem Th4:
  for F1,F2 being Functor of A,B st F1 is_transformable_to F2 for t
being transformation of F1,F2 st t is invertible for a being Object of A holds
  F1.a, F2.a are_isomorphic
proof
  let F1,F2 be Functor of A,B such that
 F1 is_transformable_to F2;
  let t be transformation of F1,F2 such that
A1: t is invertible;
  let a be Object of A;
  take t.a;
  thus thesis by A1;
end;
