This was due to:
 a) removing the implicit cycling mechanism code,
 which I did early on as part of getting ISO 8601 cycling
 to work - I didn't understand what the code was doing
 at first, and it looked like it didn't help implicit cycling
 (which of course it doesn't).
b) a bias on my part to avoid implicit cycling
 where it's unnecessary - at least in the examples and tests.