fact: Definition fresh_in (x: nat) (t: poly_type) := allfv_poly_type (fun y => x <> y) t.
imports: List Lia Undecidability.SystemF.SysF Undecidability.SystemF.Autosubst.syntax Undecidability.SystemF.Autosubst.unscoped Undecidability.SystemF.Util.Facts ssreflect ssrbool ssrfun