@ensure_connection<EOL><INDENT>def begin(self, access_mode=None):<DEDENT>
if self._active_transaction:<EOL><INDENT>raise SystemError("<STR_LIT>")<EOL><DEDENT>self._active_transaction = self.driver.session(access_mode=access_mode).begin_transaction()<EOL>
Begins a new transaction, raises SystemError exception if a transaction is in progress