🚀 go-pugleaf

I'm a long time Zmacs user who has been using Epoch a couple weeks. 
I'd like to be able to see the region from mark to point highlighted
whenever mark is non-nil, so I wrote a little function to make a zone
from mark to point with a style that gives the zone a different color.

The problem is that I don't know how to get the function to be called
each time point changes.  Is there some hook that I haven't found?  Or
am I maybe taking the wrong approach entirely? 

Any help on this will be much appreciated.

  -- Garr