[plt-scheme] Datalog questions

Is there any way to:

Stop at the first answer to a query, rather than print all answers to a query?

Show some sort of trace of all the things that are being tried and
processed to resolve a query?

Express a not-equals constraint, such as:
link(2,Y) :- link(1,Y), Y!=3.
where such a rule would mean that you can deduce link(2,1) from
link(1,1) but not link(2,3) from link(1,3).

Thanks.