code: function(block, line_number) {
    var b = block;
    var last_list = null;
    do {
      if (b.t === 'List') {
        last_list = b;
      }
      b = b.parent;
    } while (b);
  
    if (last_list) {
      while (block != last_list) {
        this.finalize(block, line_number);
        block = block.parent;
      }
      this.finalize(last_list, line_number);
      this.tip = last_list.parent;
    }
  }
docs: Break out of all containing lists, resetting the tip of the document to the parent of the highest list, and finalizing all the lists.  (This is used to implement the "two blank lines break of of all lists" feature.)