fact: Definition functional X Y (f : X -> Y -> Prop) := forall x y1 y2, f x y1 -> f x y2 -> y1 = y2.
imports: Arith Lia List Bool Eqdep_dec Undecidability.Shared.Libs.DLW(utils_tac utils_nat utils_list finite pos vec) Undecidability.MuRec.MuRec