+(* if one half of an iso is central, so is the other half *)
+Lemma iso_central_both `{C:BinoidalCat}{a b:C}(i:a ≅ b) : CentralMorphism #i -> CentralMorphism #i⁻¹.
+ intro cm.
+ apply Build_CentralMorphism; intros; simpl.
+ etransitivity.
+ symmetry.
+ apply right_identity.
+ setoid_rewrite associativity.
+ setoid_replace (id (a ⊗ d)) with ((#i >>> iso_backward i) ⋉ d).
+ setoid_rewrite <- fmor_preserves_comp.
+ setoid_rewrite <- associativity.
+ setoid_rewrite juggle3.
+ setoid_rewrite <- (centralmor_first(CentralMorphism:=cm)).
+ setoid_rewrite <- associativity.
+ setoid_rewrite (fmor_preserves_comp (-⋉c)).
+ apply comp_respects; try reflexivity.
+ etransitivity; [ idtac | apply left_identity ].
+ apply comp_respects; try reflexivity.
+ etransitivity; [ idtac | apply (fmor_preserves_id (-⋉c)) ].
+ apply (fmor_respects (-⋉c)).
+ apply iso_comp2.
+ symmetry.
+ etransitivity; [ idtac | apply (fmor_preserves_id (-⋉d)) ].
+ apply (fmor_respects (-⋉d)).
+ apply iso_comp1.
+
+ etransitivity.
+ symmetry.
+ apply left_identity.
+ setoid_replace (id (c ⊗ b)) with (c ⋊ (iso_backward i >>> #i)).
+ setoid_rewrite <- fmor_preserves_comp.
+ setoid_rewrite juggle3.
+ setoid_rewrite <- (centralmor_second(CentralMorphism:=cm)).
+ setoid_rewrite associativity.
+ apply comp_respects; try reflexivity.
+ setoid_rewrite associativity.
+ setoid_rewrite (fmor_preserves_comp (d⋊-)).
+ etransitivity; [ idtac | apply right_identity ].
+ apply comp_respects; try reflexivity.
+ etransitivity; [ idtac | apply (fmor_preserves_id (d⋊-)) ].
+ apply (fmor_respects (d⋊-)).
+ apply iso_comp1.
+ symmetry.
+ etransitivity; [ idtac | apply (fmor_preserves_id (c⋊-)) ].
+ apply (fmor_respects (c⋊-)).
+ apply iso_comp2.
+ Qed.
+