 pool object will currently
run by the value of &rsquo;delta&rsquo;.  The main purpose of this is to enable a
task to increment the maximum thread number where it is about to enter
a call which may block (non-asynchronously) for some time, with a view
to decrementing it later when it has finished making blocking calls,
so as to enable another thread to keep a core active.  A
with-thread-pool-increment macro is available which will do this for
you automatically in an exception-safe way (see the documentation