Definition good_op_move (x y : op) : Prop :=
    (forall k k', ~ (x = O k /\ y = I k')) /\
    (forall k, ~ (x = I k /\ y = O k)).

Lemma op_equiv_app_tail : forall l xs ys, op_equivalent xs ys -> op_equivalent (xs ++ l) (ys ++ l).