fact: Definition free' sigma := (forall x, bound x -> sigma x = var x) /\ (forall x, free x -> forall y, y ∈ vars (sigma x) -> free y).
imports: List Lia Arith Init.Wf Morphisms Program.Program Undecidability.HOU(concon.conservativity calculus.calculus) Undecidability.HOU.unification(systemunification nth_order_unification)