docs: private String parseParamsForSystemCatalog(ParameterSet params, JSONObject obj) throws Exception
      {
          if (params.toArray().length != 1) {
              return "Incorrect number of arguments to @SystemCatalog (expects 1, received " +
                      params.toArray().length + ")";
          }
          Object first = params.toArray()[0];
          if (!(first instanceof String)) {
              return "First argument to @SystemCatalog must be a valid STRING selector, instead was " +
                      first;
          }
          // Would be nice to have subselector validation here, maybe.  Maybe later.
          String subselector = (String)first;
          obj.put("subselector", subselector);
          obj.put("interval", false);
  
          return null;
      }