Class Equiv A := equiv : A -> A -> Prop.
Class Setoid A `{Equiv A} := setoid_equiv:> Equivalence (equiv).
Global Declare Instance vec_equiv {A} `{Equiv A} {n}: Equiv (Vector.t A n).
Global Declare Instance vec_setoid A `{Setoid A} n : Setoid (Vector.t A n).
Global Declare Instance tl_proper1 {A} `{Equiv A} n:
Proper ((equiv) ==> (equiv))
(@tl A n).

Lemma test:
forall {A} `{Setoid A} n (xa ya: Vector.t A (S n)),
(equiv xa ya) -> equiv (tl xa) (tl ya).