I think you should be able to avoid crashing if passed a non-relmapper file. Make sure not to loop over more mappings than exist in the relmapper file of the given size.
I guess you should warn if the number of mappings is too large for the file's size. And then "cap" the number of mappings to the maximum possible number.
Ah, thanks for the tip. That's right -- I can't assume the user's input is a valid file. Updated patch here.
