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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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())