intros b1 b2 sz w1 w2 ? H0; intuition.
apply (f_equal (@whd _)) in H0; tauto.
apply (f_equal (@wtl _)) in H0; tauto.
Qed.

(* Specifications: *)
Theorem weqb_true_iff : forall sz x y,
@weqb sz x y = true <-> x = y.