ady a read
watch for the file passed, the old one will be replaced by the new
one.  If &rsquo;proc&rsquo; returns #f, the read 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 be either a port or a file
descriptor.  If &rsquo;file&rsquo; is a file descriptor, any port for the
descriptor is not referenced for garbage collection purposes - it must
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