proposition: Lemma in_one : List.In x [ y ] <-> x = y.
proof: Proof. clear. simpl. intuition congruence. Qed.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.LibTactics Metalib.MetatheoryAtom Metalib.LibTactics