docs: def add_substitution(self, substitution):
          """Add a substitution to the email
  
          :param value: Add a substitution to the email
          :type value: Substitution
          """
          if substitution.personalization:
              try:
                  personalization = \
                      self._personalizations[substitution.personalization]
                  has_internal_personalization = True
              except IndexError:
                  personalization = Personalization()
                  has_internal_personalization = False
              personalization.add_substitution(substitution)
  
              if not has_internal_personalization:
                  self.add_personalization(
                      personalization, index=substitution.personalization)
          else:
              if isinstance(substitution, list):
                  for s in substitution:
                      for p in self.personalizations:
                          p.add_substitution(s)
              else:
                  for p in self.personalizations:
                      p.add_substitution(substitution)