1) Comments do not start with a ;
   Any line that doesn't start with an @ is either a parameter to
   the last command (such as in @show), or is ignored.  Further,
   "extra" arguments are also ignored.  This means you can literally
   type comments almost anywhere.
2) There is a maximum "call stack" of two "frames".  If you nest
   subroutines three deep, the first one will never be returned to.
3) Implement @routineabort.
4) If @run is used in a subroutine, it acts as an immediate return
   The arguments are not consulted, and no error is displayed.
5) Of course, if you call subroutines three deep and return from
   two of them, the first subroutine is now the topmost frame, so
   run behaves normally there.