fact: Fixpoint steps k x := match k with | 0 => Some x | S k => match mm2_sig_step_dec M x with | inleft (exist _ y _) => steps k y | inright _ => None end end.
imports: List ListDec PeanoNat Lia Relation_Operators Operators_Properties Permutation Undecidability.MinskyMachines.MM2 Undecidability.MinskyMachines.Util.MM2_facts ssreflect ssrbool ssrfun