code: def _possible_issuers(self, cert):
          
  
          issuer_hashable = cert.issuer.hashable
          if issuer_hashable not in self._subject_map:
              return
  
          for issuer in self._subject_map[issuer_hashable]:
              # Info from the authority key identifier extension can be used to
              # eliminate possible options when multiple keys with the same
              # subject exist, such as during a transition, or with cross-signing.
              if cert.authority_key_identifier and issuer.key_identifier:
                  if cert.authority_key_identifier != issuer.key_identifier:
                      continue
              elif cert.authority_issuer_serial:
                  if cert.authority_issuer_serial != issuer.issuer_serial:
                      continue
  
              yield issuer
docs: Returns a generator that will list all possible issuers for the cert
  
          :param cert:
              An asn1crypto.x509.Certificate object to find the issuer of