(* o_NTh is associative *)

Definition EqualH_NT (C D : Category) (F G F' G' : Functor C D) 
  (T : NT F G) (T' : NT F' G') := forall a : C, T a =_H T' a.Infix "=_NTH" := EqualH_NT (at level 70).

Lemma CompH_NT_assoc : (T o_NTh T') o_NTh T'' =_NTH T o_NTh T' o_NTh T''.