docs: def fetch(sequence, time='hour'):
      """
      Fetch an OpenStreetMap diff file.
  
      Parameters
      ----------
      sequence : string or integer
          Diff file sequence desired. Maximum of 9 characters allowed. The value
          should follow the two directory and file name structure from the site,
          e.g. https://planet.osm.org/replication/hour/NNN/NNN/NNN.osc.gz (with
          leading zeros optional).
  
      time : {'minute', 'hour', or 'day'}, optional
          Denotes the diff file time granulation to be downloaded. The value
          must be a valid directory at https://planet.osm.org/replication/.
  
      Returns
      -------
      data_stream : class
          A file-like class containing a decompressed data stream from the
          fetched diff file in string format.
  
      """
      import StringIO
      import gzip
      import requests
  
      if time not in ['minute','hour','day']:
          raise ValueError('The supplied type of replication file does not exist.')
  
      sqn = str(sequence).zfill(9)
      url = "https://planet.osm.org/replication/%s/%s/%s/%s.osc.gz" %\
            (time, sqn[0:3], sqn[3:6], sqn[6:9])
      content = requests.get(url)
  
      if content.status_code == 404:
          raise EnvironmentError('Diff file cannot be found.')
      
      content = StringIO.StringIO(content.content)
      data_stream = gzip.GzipFile(fileobj=content)
  
      return data_stream