proposition: Lemma reachable_n_mon' {M n n' X X' Y Y'} : n <= n' -> (X, Y) = (X', Y') -> reachable_n M n X Y -> reachable_n M n' X' Y'.
proof: Proof. move=> ? [? ?] ?. subst. by apply: reachable_n_mon; eassumption. 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