proposition: Lemma mm2_instr_at_pos {P: list mm2_instr} {mmi: mm2_instr} {i: nat} : mm2_instr_at mmi i P -> i = S (Nat.pred i).
proof: Proof. by move=> [?] [?] [_ <-] /=. Qed.
imports: List PeanoNat Lia Relation_Operators Operators_Properties Wellfounded.Transitive_Closure Undecidability.MinskyMachines.MM2 ssreflect ssrbool ssrfun