proposition: Lemma maybe_index_yes_near_futile l m i j n z : In (i, (j, n)) iP -> m <= n -> maybe_reachable 0 (l, §0^m, '#+i) z.
proof: Proof. move=> Hijn ?. apply: terminal_maybe_reachable. move=> ? /(index_yes_step_shape Hijn). case; first by case: (m). move /(f_equal (@length Symbol)). rewrite ?app_length ?repeat_length. by lia. Qed.
imports: Relation_Operators Operators_Properties Lia PeanoNat List Undecidability.StackMachines.Reductions.CM1_HALT_to_SMNdl_UB.SMX_facts Undecidability.StackMachines.Util(Nat_facts List_facts) ssreflect ssrbool ssrfun