Goal:
  
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   (∀ (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; try Tactics.discriminates