destruct l; simpl...
intros.
elimtype False.
specialize (H x).
unfold beq_X in H.
destruct (eq_X_dec x x)...
simpl in H.
discriminate.
Qed.

(* Specifications: *)
Lemma counts_0_expec_length_0 t:
(forall i, expec (count (beq_X i)) t = 0) -> expec (@length (_:Set)) t = 0.