Inductive equiv_hlist : forall ls, hlist ls -> hlist ls -> Prop :=
    | hlist_eqv_nil : equiv_hlist Hnil Hnil
    | hlist_eqv_cons : forall l ls x y h1 h2, eqv x y -> equiv_hlist h1 h2 ->
      @equiv_hlist (l :: ls) (Hcons x h1) (Hcons y h2).Inductive equiv_hlist : forall ls, hlist ls -> hlist ls -> Prop :=
    | hlist_eqv_nil : equiv_hlist Hnil Hnil
    | hlist_eqv_cons : forall l ls x y h1 h2, eqv x y -> equiv_hlist h1 h2 ->
      @equiv_hlist (l :: ls) (Hcons x h1) (Hcons y h2).Inductive equiv_hlist : forall ls, hlist ls -> hlist ls -> Prop :=
    | hlist_eqv_nil : equiv_hlist Hnil Hnil
    | hlist_eqv_cons : forall l ls x y h1 h2, eqv x y -> equiv_hlist h1 h2 ->
      @equiv_hlist (l :: ls) (Hcons x h1) (Hcons y h2).Global Instance Reflexive_equiv_hlist (R : forall t, Reflexive (@eqv t)) ls
    : Reflexive (@equiv_hlist ls).Global Instance Symmetric_equiv_hlist (R : forall t, Symmetric (@eqv t)) ls
    : Symmetric (@equiv_hlist ls).Global Instance Transitive_equiv_hlist (R : forall t, Transitive (@eqv t)) ls
    : Transitive (@equiv_hlist ls).

Lemma equiv_hlist_app : forall a b (c c' : hlist a) (d d' : hlist b), (equiv_hlist c c' /\ equiv_hlist d d') <-> equiv_hlist (hlist_app c d) (hlist_app c' d').