proc&rsquo; callback should take a single argument, and when called this
will be set to &rsquo;out or &rsquo;excpt.  The same port or file descriptor can
also be passed to event-loop-add-read-watch, and if so and the
descriptor is also available for reading or in exceptional condition,
the read callback will also be called with its argument set to &rsquo;in or
&rsquo;excpt (if both a read and a write watch have been set for the same
file argument, and there is an exceptional condition, it is the read
watch 