docs: def build_routes(pfeed):
      """
      Given a ProtoFeed, return a DataFrame representing ``routes.txt``.
      """
      f = pfeed.frequencies[['route_short_name', 'route_long_name',
        'route_type', 'shape_id']].drop_duplicates().copy()
  
      # Create route IDs
      f['route_id'] = 'r' + f['route_short_name'].map(str)
  
      del f['shape_id']
  
      return f