docs: def add_to(self, email):
          """Add a single recipient to this Personalization.
  
          :type email: Email
          """
          if email.substitutions:
              if isinstance(email.substitutions, list):
                  for substitution in email.substitutions:
                      self.add_substitution(substitution)
              else:
                  self.add_substitution(email.substitutions)
          if email.subject:
              if isinstance(email.subject, str):
                  self.subject = email.subject
              else:
                  self.subject = email.subject.get()
          self._tos.append(email.get())