def close(self, reply_code=0, reply_text='', method_sig=(0, 0)):
        """
        request a channel close

        This method indicates that the sender wants to close the
        channel. This may be due to internal conditions (e.g. a forced
        shut-down) or due to an error handling a specific method, i.e.
        an exception.  When a close is due to an exception, the sender
        provides the class and method id of the method which caused
        the exception.

        RULE:

            After sending this method any received method except
            Channel.Close-OK MUST be discarded.

        RULE:

            The peer sending this method MAY use a counter or timeout
            to detect failure of the other peer to respond correctly
            with Channel.Close-OK..

        PARAMETERS:
            reply_code: short

                The reply code. The AMQ reply codes are defined in AMQ
                RFC 011.

            reply_text: shortstr

                The localised reply text.  This text can be logged as an
                aid to resolving issues.

            class_id: short

                failing method class

                When the close is provoked by a method exception, this
                is the class of the method.

            method_id: short

                failing method ID

                When the close is provoked by a method exception, this
                is the ID of the method.

        """
        if not self.is_open:
            # already closed
            return

        args = AMQPWriter()
        args.write_short(reply_code)
        args.write_shortstr(reply_text)
        args.write_short(method_sig[0]) # class_id
        args.write_short(method_sig[1]) # method_id
        self._send_method((20, 40), args)
        return self.wait(allowed_methods=[
                          (20, 41),    # Channel.close_ok
                        ])