Generate the tree lazily.

[blinks]

Increasing depths cause larger and larger delays at the beginning. Oberon should generate just enough metadata to guide the tree through an arc before descending, minimizing startup time.

I'm almost certain something in the code is forcing execution -- probably duration, which uses the last event in a list to determine the total length. If I could avoid that (perhaps by prescribing duration instead of retrieving it), it'd be easy to keep it lazy.