code: @VisibleForTesting
    boolean isIntact() {
      for (int i = 1; i < size; i++) {
        if (!heapForIndex(i).verifyIndex(i)) {
          return false;
        }
      }
      return true;
    }
docs: Returns {@code true} if the MinMax heap structure holds. This is only used in testing.
  
  <p>TODO(kevinb): move to the test class?