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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 addPrivateCertificate(self, subjectName, existingCertificate=None):
 """ Add a PrivateCertificate object to this store for this subjectName. If existingCertificate is None, add a new self-signed certificate. """

        if existingCertificate is None:
            assert '@' not in subjectName, "Don't self-sign user certs!"
            mainDN = DistinguishedName(commonName=subjectName)
            mainKey = KeyPair.generate()
            mainCertReq = mainKey.certificateRequest(mainDN)
            mainCertData = mainKey.signCertificateRequest(
                mainDN, mainCertReq,
                lambda dn: True,
                self.genSerial(subjectName)
            )
            mainCert = mainKey.newCertificate(mainCertData)
        else:
            mainCert = existingCertificate
        self.localStore[subjectName] = mainCert