protected void testClear(int N) {
   int[] keys = new int[N]; // Slice is 1000.
   for (int i = 0; i < keys.length; i++) keys[i] = i;
   RangeIndex rIndex = makeRangeIndex(3);
   add(rIndex, keys);
   if (N > 0) assertFalse(rIndex.isEmpty());
   rIndex.clear();
   assertTrue(rIndex.isEmpty());
 }