Hmmmm.

Maybe like this:

+  +  +  +  +       +  +
|  |  |  |  |       |  |
|  |  |  |  |       |  |
|  |  |  |  |.......|  |
|  |  |  |  |       |  |
|  |  |  |  |       |  |
+  +  +  +  +       +  +

+--+--+--+--+.......+--+

Where mostly we run around the bottom alley, heedless of 'levels';
or if need be, move up and start to hop over those little vertical 
fences when we need to keep the metalevels straight?

I wonder how to make this drawing out of software.  

I have the thesis with the 3-lisp implementation in it around 
here some place.  That might be a good place to look.

It certainly knew how to make the fences, but no alley, I think.

-- 
+---Tom Morgan ------------------------------ (tmorgan at acm.org) -----+
I   Brooklyn Union                          Phone: 1 718 403 2427    I
I   1 MetroTech Center                      Fax:   1 718 488 1752    I
+---Brooklyn, New York 11201 USA ------------------------------------+