`ds_step` could be negative if the element length is negative.

Right now it is assumed that a negative `ds_step` means that this parameter has not been set. This will be problematic with negative length elements. Possible solutions are to use zero as meaning that `ds_step` has not been set or something like `undef`. Anyone have any comments?

Start by locating the implementation and uses of ds_step, then read how negative element lengths are represented and how an unset ds_step is detected. The issue does not name files or tests; done requires choosing and applying a sentinel that distinguishes an unset value from a valid negative step, with coverage for negative-length elements.

Written by the indexing model from the issue text.