proposition: Lemma one_mid_neq_nil : xs ++ one x ++ ys <> nil.
proof: Proof. clear. intros H. symmetry in H. auto using nil_neq_one_mid. 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