Definition eqb:
   forall {m n} (v1: t A m) (v2: t A n), bool :=
   fix fix_beq {m n} v1 v2 :=
   match v1, v2 with
     |[], [] => true
     |_ :: _, [] |[], _ :: _ => false
     |h1 :: t1, h2 :: t2 => A_beq h1 h2 && fix_beq t1 t2
   end%bool.

Lemma eqb_nat_eq: forall m n (v1: t A m) (v2: t A n) (Hbeq: eqb v1 v2 = true), m = n.