revert a; induction l; intro a'... intro B.
cut (a = a' \/ In a' (rsort_uniq cmp l)); [intro C|].
destruct C... eapply insert_uniq_In; eauto.
Qed.

(* Specifications: *)
Lemma pick_In (cls : M.t) cl :
M.min_elt cls = Some cl -> In cl (M.elements cls).