What would be really nice here is if we had a `Method` class. We could move methods that are useful to all methods into it like `_mat_inv_mul()` into it. Also note that `_mat_inv_mul` does not use `self` so it can either be a static method or moved into the functions module. We don't want to have duplicate code to maintain if this is both in the KaneMethod and LangrangesMethod classes.