kind=defined
names=add_listener
visibility=public

--- add_listener(channel_name, listener_key=NOTHING, &listener)

Adds the passed proc as a listener on the
channel indicated by channel_name. listener_key
is used to remove the listener later; if none is
specified, the proc itself is used.

Whatever is used as the listener_key is
returned, making it very easy to use the proc
itself as the listener_key:

 listener = add_listener("Channel") { ... }
 remove_listener("Channel", listener)

