anchor.diff:          }
       ;
   
  -value_list returns [Object json]
  +non_variable_value_list returns [Object json]
   @init {
       JSONArray jsonArray = new JSONArray();
   }
  
positive.code:     ;
  
  facet_param returns [String facet, JSONObject param]
      :   LPAR column_name COMMA STRING_LITERAL COMMA facet_param_type COMMA (val=value | valList=value_list) RPAR
          {
              $facet = $column_name.text; // XXX Check error here?
              try {
  
negative.code:         )* RPAR
          {
              $json = jsonArray;
          }
      ;
  
  
  value_list returns [Object json]
      :   non_variable_value_list
          {
              $json = $non_variable_value_list.json;
          }
      |   VARIABLE
          {
  