Some problems about writing new fixes

Hi folks!
I have looked at lammps developer.pdf and followed the guidence of wirting a new Fix which stores atoms position from previous time step but it didn't work! I think the code is not complete. For example, the int setmask() is a pure virtual function in the fix.h, it must be defined in child classes. But is it enough? How to complete this fix?
Thank you,


please also have a look at the information provided in the manual

but there is no need to write a fix that stores position data from the previous time step, as a fix to do this (and much more) already exists in fix store/state.