A.5 library(broadcast): Broadcast and receive event notifications
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Availability::- use_module(library(broadcast)).(can be autoloaded)
Sourcelisten(+Template, :Goal)
Register a listen channel. Whenever a term unifying Template is broadcasted, call Goal. The following example traps all broadcasted messages as a variable unifies to any message. It is commonly used to debug usage of the library. 
?- listen(Term, (writeln(Term),fail)).
?- broadcast(hello(world)).
hello(world)
true.