code: def _parse_frequencies(self):
          
          frequencies = OrderedDict([
              ('EXAC', 'Unknown'),
              ('ESP', 'Unknown'),
              ('TGP', 'Unknown')])
          pref_freq = 'Unknown'
          for source in frequencies.keys():
              freq_key = 'AF_' + source
              if freq_key in self.info:
                  frequencies[source] = self.info[freq_key]
                  if pref_freq == 'Unknown':
                      pref_freq = frequencies[source]
          return pref_freq, frequencies
docs: Parse frequency data in ClinVar VCF