* The way FILL_2D works in the rend extension is kind of messed up.
  It takes its left and right boundaries for plotting points from 
  the leftmost and rightmost cells that the view window overlaps.
  If the view window happens to overlap just a tiny bit with a
  *HUGE* open-interval cell, you end up plotting tons more
  points than you need.  This should be fixed!

* The rend extension needs to be reworked so that it uses floating
  point computation!  

* Clean-up interface commands ... many of these are simply
  irrelevant. 

