void toString()
{
head.toString();
cout << " :- ";
for (unsigned int i = 0; i < PredicateList.size(); i++)
{
PredicateList[i].toString();
if (i < PredicateList.size()-1)
cout << ",";
}
cout << ".";
}
bool Predicate::operator < (const Predicate &other) const {
return (toString() < other.toString());
}