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

[DavidSagan]

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?

[JosephPeterDevlin]

What is a negative-length element used for?

[DavidSagan]

You will find this in some lattices typically to get around some limitation of the simulation program. Generally it is a negative length drift. For example, from the earliest days of Bmad, for the CESR ring, to simulate a BPM that was tucked just inside a quadrupole, a negative length drift was used.