proposition: Lemma reachable_n_step {M X Y} : step M X Y -> reachable_n M 1 X Y.
proof: Proof. move=> ?. apply: reach_step; [by eassumption | by apply: reach_refl]. Qed.
imports: List Lia Relation_Operators Undecidability.StackMachines.Util(Nat_facts List_facts) Undecidability.StackMachines.Reductions.CM1_HALT_to_SMNdl_UB.SMX ssreflect ssrbool ssrfun