- Should limit the size of the buffer in TextBufferWindow.  It's
pretty memory intensive; lot of layout objects.

- Performance, particularly on Mac OS X.  (Of course, Apple should do
something about their abyssmal Java performance, in general.)

          - Profiling suggets that the machine wastes a lot of time
          decoding operands.  I use the same algorithm Andrew P. does
          now.  Maybe a JIT?  (That's a lot of work...)

          - As for zing performance, that's even tougher.  Can
          probably instantiate fewer objects, keep the GC at bay.
          Can implement faster layout just for line input in
          TextBufferWindow.  

- There should be a way to specify a single font for all buffer window
styles and one for all grid window styles.

- Should implement the Unicode extension

? Macros, like in Andrew Plotkin's interpreters

? Cut, copy, and paste:  big pain to implement, and I don't use it...