public object VisitQuantifiedSentence(QuantifiedSentence sentence,
object arg)
{
ISentence quantified = sentence.Quantified;
var universalScope = (ISet <Variable>)arg;
// Skolemize: Skolemization is the process of removing existential
// quantifiers by elimination. This is done by introducing Skolem
// functions. The general rule is that the arguments of the Skolem
// function are all the universally quantified variables in whose
// scope the existential quantifier appears.
if (Quantifiers.IsExists(sentence.Quantifier))
{
IDictionary <Variable, ITerm> skolemSubst = new Dictionary <Variable, ITerm>();
foreach (Variable eVar in sentence.Variables)
{
if (universalScope.Count > 0)
{
// Replace with a Skolem Function
var skolemFunctionName = parser.GetFOLDomain()
.AddSkolemFunction();
skolemSubst[eVar] = new Function(skolemFunctionName, new List <ITerm>((IEnumerable <ITerm>)universalScope));
}
else
{
// Replace with a Skolem Constant
String skolemConstantName = parser.GetFOLDomain()
.AddSkolemConstant();
skolemSubst[eVar] = new Constant(skolemConstantName);
}
}
ISentence skolemized = substVisitor.Subst(skolemSubst, quantified);
return(skolemized.Accept(this, arg));
}
// Drop universal quantifiers.
if (Quantifiers.IsForall(sentence.Quantifier))
{
// Add to the universal scope so that
// existential skolemization may be done correctly
universalScope.UnionWith(sentence.Variables);
ISentence droppedUniversal = (ISentence)quantified.Accept(this, arg);
// Enusre my scope is removed before moving back up
// the call stack when returning
foreach (var v in sentence.Variables)
{
universalScope.Remove(v);
}
return(droppedUniversal);
}
// Should not reach here as have already
// handled the two quantifiers.
throw new InvalidOperationException("Unhandled Quantifier:" + sentence.Quantifier);
}
public Literal CreateAnswerLiteral(ISentence forQuery)
{
string alName = parser.GetFOLDomain().AddAnswerLiteral();
IList <ITerm> terms = new List <ITerm>();
ISet <Variable> vars = variableCollector.CollectAllVariables(forQuery);
foreach (Variable v in vars)
{
// Ensure copies of the variables are used.
terms.Add((ITerm)v.Copy());
}
return(new Literal(new Predicate(alName, terms)));
}