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Make arg/3 work according to spec

--- a/builtins.c

+++ b/builtins.c

@@ -372,18 +372,23 @@

 	Term *term = n->next;

 	Term *arg = term->next;

+	if(n->tag == VariableTerm || term->tag == VariableTerm)

+		Throw(instantiationerror());

 	if(n->tag != IntegerTerm)

-		return 0;

+		Throw(typeerror(L"integer", n));

+	if(term->tag != CompoundTerm)

+		Throw(typeerror(L"compound", term));

 	if(n->ival < 0)

 		Throw(domainerror(L"not_less_than_zero", n));

-	if(term->tag != CompoundTerm)

+

+	if(n->ival > term->arity)

 		return 0;

-	if(n->ival >= term->arity)

-		return 0;

+	print("Passed checks!\n");

+

 	int i;

 	Term *t;

-	for(i = 0, t = term->children; i < n->ival; i++, t = t->next);

+	for(i = 1, t = term->children; i < n->ival; i++, t = t->next);

 	return unify(arg, t, bindings);

 }