The library does not support sparse matrices.
For sparse matrices there is going to be some kind of structure vector. It is going to have to be calculated for the results of expressions in much the same way that types are calculated. In addition, a whole new set of row and column operations would have to be written.
Sparse matrices are important for people solving large sets of differential equations as well as being important for statistical and operational research applications.
But there are packages being developed specifically for sparse matrices and these might present the best approach, at least where sparse matrices are the main interest.