docs: def subject(self, value):
          """The subject of the email(s)
  
          :param value: The subject of the email(s)
          :type value: Subject, string
          """
          if isinstance(value, Subject):
              if value.personalization is not None:
                  try:
                      personalization = \
                          self._personalizations[value.personalization]
                      has_internal_personalization = True
                  except IndexError:
                      personalization = Personalization()
                      has_internal_personalization = False
                  personalization.subject = value.subject
  
                  if not has_internal_personalization:
                      self.add_personalization(
                          personalization,
                          index=value.personalization)
              else:
                  self._subject = value
          else:
              self._subject = Subject(value)