🚀 go-pugleaf

> >> >Consider the following trivial predicate:
> >> >a([]).
> >> >a(_).
> >> >
> >> >Given the query :-a([]). , C-Prolog finds one match and UNSW Prolog finds two.
> >> >Which is standard behavior?  How do other implementations behave?
> >-------------
> >Hmmm.  The reason for logic programming's existence is to dispense with
> >guesses about what behaviour should be.  The formulae assert that the
> >individual constant named `[]' and everthing else (i.e., `_') is in the 
> >class named by the predicate `a'.  If you believe that the anonymous variable
> >is a universially quantified variable, then there are two resolution proofs
> >of the query a([]).
> >
> >Implementors' treatment of `_' can produce non-standard behaviour; non-standard
> >means not consistent with the logical interpretation.
> 
> I have to disagree, Randy.  The query merely asks whether or not 
> there is a proof of "a([])".  Not how many there are.
--------------------------------------------------------------------
There is nothing in my reply that says anything about what an implementation
should do with such proofs (count them, print them, ignore them, etc.).  All
it says is that there are two proofs.  If you ask for all proofs, then there
had better be two.  8-).