fact: Variables (s t: exp X) (x: nat) (sigma: fin -> exp X) (S: list (exp X)).
imports: RelationClasses Morphisms List Arith Lia Init.Nat Setoid Undecidability.HOU(std.std calculus.calculus unification.unification) Undecidability.HOU(third_order.pcp third_order.encoding)