fact: Definition symbols := (flat_map (fun '((a,b),(c,d)) => [a; b; c; d]) rs) ++ [0; 1].
imports: Undecidability.IntersectionTypes.CD Undecidability.IntersectionTypes.Util.CD_facts Undecidability.StringRewriting.SSTS Undecidability.LambdaCalculus.Util.term_facts Relations List PeanoNat Lia ssreflect ssrfun