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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_uid(self, uid, selfsign=True, **prefs):
 """ Add a User ID to this key. :param uid: The user id to add :type uid: :py:obj:`~pgpy.PGPUID` :param selfsign: Whether or not to self-sign the user id before adding it :type selfsign: ``bool`` Valid optional keyword arguments are identical to those of self-signatures for :py:meth:`PGPKey.certify`. Any such keyword arguments are ignored if selfsign is ``False`` """

        uid._parent = self
        if selfsign:
            uid |= self.certify(uid, SignatureType.Positive_Cert, **prefs)

        self |= uid