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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 buffer(self):
 """ Context manager to temporarily buffer the output. :raise RuntimeError: If two :meth:`buffer` context managers are used nestedly. If the context manager is left without exception, the buffered output is sent to the actual sink. Otherwise, it is discarded. In addition to the output being buffered, buffer also captures the entire state of the XML generator and restores it to the previous state if the context manager is left with an exception. This can be used to fail-safely attempt to serialise a subtree and return to a well-defined state if serialisation fails. :meth:`flush` is not called automatically. If :meth:`flush` is called while a :meth:`buffer` context manager is active, no actual flushing happens (but unfinished opening tags are closed as usual, see the `short_empty_arguments` parameter). """

        if self._buf_in_use:
            raise RuntimeError("nested use of buffer() is not supported")
        self._buf_in_use = True
        old_write = self._write
        old_flush = self._flush

        if self._buf is None:
            self._buf = io.BytesIO()
        else:
            try:
                self._buf.seek(0)
                self._buf.truncate()
            except BufferError:
                # we need a fresh buffer for this, the other is still in use.
                self._buf = io.BytesIO()

        self._write = self._buf.write
        self._flush = None
        try:
            with self._save_state():
                yield
            old_write(self._buf.getbuffer())
            if old_flush:
                old_flush()
        finally:
            self._buf_in_use = False
            self._write = old_write
            self._flush = old_flush