code: public static WithinGroup<Object> listagg(Expression<?> expr, String delimiter) {
          return new WithinGroup<Object>(Object.class, SQLOps.LISTAGG, expr, ConstantImpl.create(delimiter));
      }
docs: LISTAGG orders data within each group specified in the ORDER BY clause and then concatenates
  the values of the measure column.
  
  @param expr measure column
  @param delimiter delimiter
  @return listagg(expr, delimiter)