<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def filter(self, dataset):
 """Mark unusual trips for all the routes in the dataset."""

    self.info('Going to filter infrequent routes in the dataset')
    for route in dataset.routes.values():
      self.filter_line(route)