code: def add_tls_credential(self, credentials):
          
          for cred in credentials:
              href = element_resolver(cred)
              if href not in self.engine.server_credential:
                  self.engine.server_credential.append(href)
docs: Add a list of TLSServerCredential to this engine.
          TLSServerCredentials can be in element form or can also
          be the href for the element.
          
          :param credentials: list of pre-created TLSServerCredentials
          :type credentials: list(str,TLSServerCredential)
          :return: None