code: function move(node, stayWithin, directionIsNext) {
    if (directionIsNext) {
      return NodeTraversal.next(node, stayWithin);
    } else {
      if (node === stayWithin) {
        return null;
      }
      return NodeTraversal.previous(node, null);
    }
  }
docs: /* Private methods and helpers 
  @based on WebKit's NodeIterator::moveToNext and NodeIterator::moveToPrevious
  https://trac.webkit.org/browser/trunk/Source/WebCore/dom/NodeIterator.cpp?rev=186279#L51