procedure which will be called with &rsquo;excpt rather than the write
watch procedure, so if that procedure returns #f only the read watch
will be removed).  If there is already a write watch for the file
passed, the old one will be replaced by the new one.  If &rsquo;proc&rsquo;
returns #f, the write watch will be removed from the event loop,
otherwise the watch will continue.  This is thread safe - any thread
may add a watch, and the callback will execute in the event loop
thread.  The file argument can 