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Add arg/3

--- a/builtins.c

+++ b/builtins.c

@@ -21,6 +21,7 @@

 BuiltinProto(builtinstring);

 BuiltinProto(builtincompare);

 BuiltinProto(builtinfunctor);

+BuiltinProto(builtinarg);

 int compareterms(Term *, Term *);

@@ -72,6 +73,8 @@

 		return builtincompare;

 	if(Match(L"functor", 3))

 		return builtinfunctor;

+	if(Match(L"arg", 3))

+		return builtinarg;

 	return nil;

 }

@@ -336,3 +339,27 @@

 	}

 	return 0;

 }

+

+int

+builtinarg(Term *database, Term *goal, Goal **goals, Choicepoint **choicestack, Binding **bindings)

+{

+	USED(database);

+	USED(goals);

+	USED(choicestack);

+

+	Term *n = goal->children;

+	Term *term = n->next;

+	Term *arg = term->next;

+

+	if(n->tag != NumberTerm || n->numbertype != NumberInt || n->ival < 0)

+		return 0;

+	if(term->tag != CompoundTerm)

+		return 0;

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

+		return 0;

+

+	int i;

+	Term *t;

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

+	return unify(arg, t, bindings);

+}

\ No newline at end of file