proposition: Lemma nil_neq_one_mid : nil <> l1 ++ [ x ] ++ l2.
proof: Proof. simpl. apply List.app_cons_not_nil. Qed.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.LibTactics Metalib.MetatheoryAtom Metalib.LibTactics