fact: Axiom fresh_mid_head : uniq (F ++ (x ~ a) ++ E) -> ~ In x (dom F).
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.AssocList Metalib.MetatheoryAtom Metalib.LibTactics