I just realised the implications of this condition. Here you're checking if one is a sequence of sequences and the other is an indicator matrix. What if the one was an array of sequences? I know at the moment that's not allowed by `is_multilabel`, but I'm not sure we want to write code that would fail if `is_multilabel` is changed to accept it.

Accepting an array of sequences in the future will mean we can perform fast vectorized operations. Note that's the format used in `scipy.sparse.lil_matrix.rows`, which is essentially the same data structure that we're working with.

So I think we need a helper function to do this `multilabel_compatible` check.