zmidi-basic

Copperbox revision 2422.

More foundational work. I've got the first object in place Event which is the analogue of Image in Wumpus-Basic. Because representing music has more structure (syntax) than graphics, the work is rather tentative and may be heavily revised.

For graphics superimposition is perfectly fine - graphics can be stacked on top of each other and the renderer uses the "painter's algorithm" or similar to draw only what is on top. My feeling is that superimposition would work for music if my primitive composition was additive synthesis (this would be equivalent to rendering with transparency rather than strictly the painter's algorithm), but as I'm composing MIDI I have to be more careful about overlapping.

For MIDI, I solve overlapping by adding overlays and voices to the intermediate syntax. This extra syntax gives me the ability to be fairly careful about contention for note-on note-offs, but having more syntax naturally makes it more complicated to actually build music - it stops me writing an `at` combinator that enjoys the (near) universality it enjoys in Wumpus.