Module Hom21 := Homomorphism N2 N1.
Local Notation h12 := Hom12.natural_isomorphism.
Local Notation h21 := Hom21.natural_isomorphism.
Local Notation "n == m" := (N1.eq n m) (at level 70, no associativity).

Lemma inverse_nat_iso : forall n : N1.t, h21 (h12 n) == n.