public Proposition makeGround() {
PropositionImpl ground = (PropositionImpl) this.clone();
for(Term term : ground.getTerms()) {
if(term.isVar() && !term.isGround()) {
Term t = new Atom(term.toString().toLowerCase());
jason.asSemantics.Unifier un = new jason.asSemantics.Unifier();
if(un.unifies(t, term)) {
term.apply(un);
}
}