Deleting Threads
----------------
A thread may be deleted by either returning from the function
specified when it was created, or by explicit deletion. Also, the
implicit thread created by the `call-with-dynamic-root' function may be
deleted by exiting from the function called in the new context.
- Function: thread-delete #!optional thread
Mark THREAD (or the current thread), as being deleted. It will not
be switched to in the future. If the current thread is deleted,
control will be passed to the next runnable thread. Deleting the
last runnable thread results forces the containing dynamic root to
be closed.
- Function: thread-deleted-p thread
Returns true if THREAD has been deleted.