docs: def get_cif(code, mmol_number, outfile=None):
      """
      Parameters
      ----------
      code : str
          PDB code.
      mmol_number : int
          mmol number (biological assembly number) of file to download. Numbers from PDBe.
          If None, defaults to the preferred biological assembly listed for code on the PDBe.
      outfile : str
          Filepath. Writes returned value to this file.
  
      Returns
      -------
      cif_string : str, or None
          Content of the cif file as a string.
          None if unable to download the cif_file from the pdbe site.
      """
      pdbe_url = "http://www.ebi.ac.uk/pdbe/static/entry/download/{0}-assembly-{1}.cif.gz".format(code, mmol_number)
      r = requests.get(pdbe_url)
      if r.status_code == 200:
          temp_gz = tempfile.NamedTemporaryFile()
          temp_gz.write(r.content)
          with gzip.open(temp_gz.name, 'rb') as foo:
              cif_string = foo.read().decode()
      else:
          print("Could not download cif file for {0}".format(code))
          return None
      # Write to file.
      if outfile and cif_string:
          with open(outfile, 'w') as foo:
              foo.write(cif_string)
      return cif_string