proposition: Lemma path_length {k x} : length (path k x) = k.
proof: Proof. by rewrite /path map_length seq_length. Qed.
imports: List ListDec PeanoNat Lia Relation_Operators Operators_Properties Permutation Undecidability.MinskyMachines.MM2 Undecidability.MinskyMachines.Util.MM2_facts ssreflect ssrbool ssrfun