Section assoc_horz_comp.
Variables (A B C D : Category) (F G : Functor A B)
(F' G' : Functor B C) (F'' G'' : Functor C D).
Variables (T : NT F G) (T' : NT F' G') (T'' : NT F'' G'').

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