sumTwoPolys

/**
     * Subtracts two polynomial linked lists.
     */
    public ObjecList subTwoPolys (ObjectList list1, ObjectList list2) {
        ObjectListNode p = list1.getFirstNode();
        ObjectListNode q = list2.getFirstNode();

        while (p != null && q != null) {
            Term t1 = (Term) p.getInfo();
            Term t2 = (Term) q.getInfo();
            if (t1.getExp() == t2.getExp()) {
                int coeff = t1.getCoeff() + t2.getCoeff();
                if (coeff != 0) {
                    attach(coeff, t1.getExp());
                }
                p = p.getNext();
                q = q.getNext();
            } else if (t1.getExp() > t2.getExp()) {
                attach(t1.getCoeff(), t1.getExp());
                p = p.getNext();
            } else {
                attach(t2.getCoeff(), t2.getExp());
                q = q.getNext();
            }
        }
        while (p != null) {
            Term t1 = (Term) p.getInfo();
            attach(t1.getCoeff(), t1.getExp());
            p = p.getNext();
        }
        while (q != null) {
            Term t2 = (Term) q.getInfo();
            attach(t2.getCoeff(), t2.getExp());
            q = q.getNext();
        }
        return sum;
    }