code: function expand(edit, frag, child, bitmap, subNodes) {
      var arr = [];
      var bit = bitmap;
      var count = 0;
      for (var i = 0; bit; ++i) {
          if (bit & 1) arr[i] = subNodes[count++];
          bit >>>= 1;
      }
      arr[frag] = child;
      return ArrayNode(edit, count + 1, arr);
  }
docs: /* Internal node operations.
  **************************************************************************** 
  Expand an indexed node into an array node.
  
  @param edit Current edit.
  @param frag Index of added child.
  @param child Added child.
  @param mask Index node mask before child added.
  @param subNodes Index node children before child added.