Goal:
  
  ============================
   (∀ (A : Type) (l : list A), {n : nat | n < #|l|} → [] = l → A)


DEBUG:Going to execute:
simpl; intros; Tactics.destruct_all_rec_calls;
            repeat (Tactics.destruct_conjs; simpl proj1_sig in *); subst *;
            Tactics.autoinjections