docs: 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