I really would prefer not to handle stored zeros in sparse routines provided in Base. Stuff like `nnz` then has to count zeros every time rather than give a constant time answer. Most routines will probably just work.

This should not prevent anyone from using stored zeros in your own codes, but it is best to squeeze the zeros out when handing over the matrices to the generic sparse matrix routines in Base.

Let's move this discussion to a new issue, as it is an important design decision.