code: public List<Field> specFields () {
          List<Field> specFields = new ArrayList<>();
          for (Field f : fields) if (f.requirement == REQUIRED || f.requirement == OPTIONAL) specFields.add(f);
          return specFields;
      }
docs: Get only those fields included in the official GTFS specification for this table, i.e., filter out fields used
  in the editor or extensions.