<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def close(self):
 """ Close the XML stream and the underlying transport. This gracefully shuts down the XML stream and the transport, if possible by writing the eof using :meth:`asyncio.Transport.write_eof` after sending the stream footer. After a call to :meth:`close`, no other stream manipulating or sending method can be called; doing so will result in a :class:`ConnectionError` exception or any exception caused by the transport during shutdown. Calling :meth:`close` while the stream is closing or closed is a no-op. """

        if (self._smachine.state == State.CLOSING or
                self._smachine.state == State.CLOSED):
            return
        self._writer.close()
        if self._transport.can_write_eof():
            self._transport.write_eof()
        if self._smachine.state == State.STREAM_HEADER_SENT:
            # at this point, we cannot wait for the peer to send
            # </stream:stream>
            self._close_transport()
        self._smachine.state = State.CLOSING