-/* It might be possible to squeeze this structure further. As best I can tell
- itermax and iterlval are never used at the same time, so it might be possible
- to make them into a union. However, I'm not confident that there are enough
- flag bits/NULLable pointers in this structure alone to encode which is
- active. There is, however, U8 of space free in struct block, which could be
- used. Right now it may not be worth squeezing this structure further, as it's
- the largest part of struct block, and currently struct block is 64 bytes on
- an ILP32 system, which will give good cache alignment.
-*/