proposition: Lemma nil_neq_one_mid : nil <> xs ++ one x ++ ys.
proof: Proof. clear. induction xs; simpl_alist; intros J; inversion J. Qed.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Coq.Strings.String String Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.LibTactics Metalib.CoqFSetInterface Coq.Classes.Equivalence Coq.Classes.EquivDec Metalib.CoqEqDec