Function threads 

pub fn threads() -> usize

The thread count parallel stages will use.

The count given to set_threads when there is one; otherwise OGEOM_THREADS from the environment when it parses as a positive count; otherwise the machine’s available parallelism, or the lent Pool’s worker count on a target that cannot start a thread. The environment is read once, on the first call that needs it. One on a target that cannot start a thread and has no pool.