Can you ask questions to find out:
baltic and the skagerrak?
black_sea, and has a capital which is
on the dimbo_vita?
There is another fact in this knowledge base, called flows_to.
This is defined as follows:
The idea it expresses is that river X flows to river or sea Y if the
water from X flows directly or indirectly to Y. Try the question
flows_to(X,Y).
and compare it with the results from
linked_to(X,Y)..
Translate the definition into good English (using pronouns instead
of variables). Why do you think it works? flows_to is
your first example of a technique, very common in
logic programming, called recursion,
where something is defined in
terms of itself. If you have studied grammars in linguistics, or
programmed in high-level languages before, you may have encountered it
already. We shall see several more examples.