// Link l's right to r; r's left to l
 private void linkLeftRight(LN<E> l, LN<E> r) {
   l.right = r;
   r.left = l;
 }
 // Link u's down to d; d's up to u
 private void linkUpDown(LN<E> u, LN<E> d) {
   u.down = d;
   d.up = u;
 }