Fix an OPNF bug that was uncovered by speedChess.

There was a non-deterministic bug causing the AssignmentIterator to
sometimes try to fill in a variable using a function that wasn't
really a function (because the variable was in the conjunct
multiple times).

src/org/ggp/base/util/gdl/model/assignments/IterationOrderCandidate.java

@@ -482,6 +482,9 @@ private GdlVariable getBestVariable(GdlSentence functionalSentence,
 						&& varsToAssign.contains(term))
 					candidateVars.add((GdlVariable) term);
 			}
+			//TODO: Should we just generate the candidate vars with a call to getProducibleVars?
+			Set<GdlVariable> producibleVars = functionInfo.getProducibleVars(functionalSentence);
+			candidateVars.retainAll(producibleVars);
 			//Now we look at the domains, trying to find the largest
 			GdlVariable bestVar = null;
 			int bestDomainSize = 0;