I. Bugs to fix

  1. The "extresist" code needs to extract substrate networks.  Moreover,
     the "extresist" code really needs to have the dependence on
     ext2sim removed, and instead read directly from .ext files.  The
     .sim format has no substrate connections, so this cannot be properly
     represented.  Also, there is nothing that is read from the .sim file
     that is not already present in the .ext file.

  2. "plow" should derive its rules from the DRC decks (using the new
     routines that are meant for just that sort of thing).

  3. It is possible to trick the net selection into an infinite loop in
     unusual geometry situations.  That these situations would be DRC
     errors does not excuse the infinite loop behavior.  The geometry
     required is unusual enough that this is not a high priority item.