broadcast_request(+Term)Unlike broadcast/1,
this predicate stops if an associated goal succeeds. Backtracking causes
it to try other listeners. A broadcast request is used to fetch
information without knowing the identity of the agent providing it. C.f. ``Is
there someone who knows the age of John?'' could be asked using
...,
broadcast_request(age_of('John', Age)),
If there is an agent (listener) that registered an `age-of'
service and knows about the age of `John' this question will be
answered.