anchor.diff:  */
   attribute_header returns [String attribute_header]
     : ATTR_HEADER attribute_attrs
  -    {$attribute_header = "{\"attributes\":{"+$attribute_attrs.attribute_attrs+"}}";}
  +    {$attribute_header = "\"attributes\":{"+$attribute_attrs.attribute_attrs+"}";}
   ;
   attribute_attrs returns [String attribute_attrs]
     : attribute_attr
  
positive.code: */
  category_header returns [String category]
    : CATEGORY_HEADER category_header_val
      {$category = "{\"category\":{" + $category_header_val.category + "}}"; }
  
    | CATEGORY_HEADER multi_cat1 = category_header_val
      {$category = "{\"category\" : [{"+$multi_cat1.category + "}";}
      (
        COMMA multi_cat2 = category_header_val
        {$category += ", {"+$multi_cat2.category + "}";}
      )+
      {$category += "]}";}
  ;
  category_header_val returns [String category]
    : term_attr scheme_attr class_attr (title_attr | rel_attr | location_attr | cat_attributes_attr | action_attr)*
  
negative.code: 
  link_header returns [String link_header]
    : LINK_HEADER single_val = link_header_val
      {$link_header = "\"link\":{" + $single_val.link_header_val + "}";}
  
    | LINK_HEADER multi_val1 = link_header_val
      {$link_header = "\"link\" : [{"+$multi_val1.link_header_val + "}";}
      (
        COMMA multi_val2 = link_header_val
        {$link_header += ", {"+$multi_val2.link_header_val + "}";}
      )+
      {$link_header += "]";}
  ;
  link_header_val returns [String link_header_val]
    : OPEN_PATH link_path_val CLOSE_PATH rel_attr
  