code: def limit(self, n, skip=None):
          
          if self.query.limit is not None:
              raise MonSQLException('LIMIT already defined')
  
          new_query_set = self.clone()
          new_query_set.query.limit = n
          new_query_set.query.skip = skip
  
          return new_query_set
docs: Limit the result set. However when the query set already has limit field before,
          this would raise an exception
          :Parameters:
          - n : The maximum number of rows returned
          - skip: how many rows to skip
          :Return: a new QuerySet object so we can chain operations