(* Lemma pickDrop_length_strong {A} (l1 l2 : seq (nat * G (option A))) (n m : nat) g : *)
(*   pickDrop l1 n = (m, g, l2) -> *)
(*   length l2 < length l1 \/ *)
(*   (length l2 = 0 /\ length l1 = 0) \/ *)
(*   (length l2 = 0 /\ (sum_fst l1 = 0 \/ sum_fst l1 <= n)). *)
(* Proof. *)
(*   revert l2 n m g. induction l1; simpl; intros l2 n m g H. *)
(*   - inv H; eauto. *)
(*   - destruct a as [k x]. *)
(*     destruct (n < k) eqn:heq. *)
(*     + inv H. left. now ssromega. *)
(*     +  *)
(*       destruct (pickDrop l1 (n - k)) as [[k' x'] xs'] eqn:Heq. *)
(*       inv H. eapply IHl1 in Heq. *)
(*       destruct Heq as [Hlt' | [[Heq1 Heq2] | [Heq1 [Heq2 | Heq2]]]]. *)
(*       * left. ssromega. *)
(*       * simpl. rewrite sum_fstE Heq1 Heq2. *)
(*         destruct l1; try discriminate. right. right. rewrite sum_fst_eq0P simpl. Hlt.  *)
(*         rewrite length_sum_lst in Hlt; eauto. ssromega. *)
(*       * rewrite sum_fstE in Hlt. *)
(*         rewrite Heq2 in Hlt. ssromega. *)
(* Qed. *)
(* Corollary pickDrop_length {A} (l1 l2 : seq (nat * G (option A))) (n m : nat) g : *)
(*   pickDrop l1 n = (m, g, l2) -> *)
(*   n < sum_fst l1 -> *)
(*   length l2 < length l1 \/ length l2 = 0. *)
(* Proof. *)
(*   intros H1 H2.  *)
(*   edestruct (@pickDrop_length_strong A); eauto. *)
(*   inv H; inv H0; eauto. *)
(* Qed. *)

Lemma pickDrop_sum_fst {A} (lg  : seq (nat * G (option A))) n :
sum_fst lg = 0 -> exists l, pickDrop lg n = (0, returnGen None, l) /\ sum_fst l = 0.