code: public List<Field> requiredFields () {
          // Filter out fields not used in editor (i.e., extension fields).
          List<Field> requiredFields = new ArrayList<>();
          for (Field f : fields) if (f.requirement == REQUIRED) requiredFields.add(f);
          return requiredFields;
      }
docs: Get only those fields marked as required in the official GTFS specification for this table.