use of variables as placeholders for predicates in conjunctions

[s-webber]

The following query:

?- X=true, X.

Produces the following error:

Expected an atom or a predicate but got a NAMED_VARIABLE with value: X

This is not the correct behavior - the correct behavior is for the query to evaluate successfully once.

Full stacktrace:

org.projog.core.ProjogException: Expected an atom or a predicate but got a NAMED_VARIABLE with value: X
        at org.projog.core.PredicateKey.createForTerm(PredicateKey.java:58)
        at org.projog.core.KnowledgeBase.getPredicateFactory(KnowledgeBase.java:199)
        at org.projog.core.function.compound.Conjunction.getPredicate(Conjunction.java:174)
        at org.projog.core.function.compound.Conjunction.getPredicate(Conjunction.java:163)
        at org.projog.core.function.compound.Conjunction.getPredicate(Conjunction.java:139)
        at org.projog.api.QueryResult.next(QueryResult.java:70)
        at org.projog.tools.ProjogConsole.evaluateOnce(ProjogConsole.java:183)
        at org.projog.tools.ProjogConsole.parseAndExecute(ProjogConsole.java:121)
        at org.projog.tools.ProjogConsole.run(ProjogConsole.java:75)
        at org.projog.tools.ProjogConsole.main(ProjogConsole.java:222)