def get_segment(self, indx):
        """This function returns an APDU coorisponding to a particular
        segment of a confirmed request or complex ack.  The segmentAPDU
        is the context."""
        if _debug: SSM._debug("get_segment %r", indx)

        # check for no context
        if not self.segmentAPDU:
            raise RuntimeError("no segmentation context established")

        # check for invalid segment number
        if indx >= self.segmentCount:
            raise RuntimeError("invalid segment number %r, APDU has %r segments" % (indx, self.segmentCount))

        if self.segmentAPDU.apduType == ConfirmedRequestPDU.pduType:
            if _debug: SSM._debug("    - confirmed request context")

            segAPDU = ConfirmedRequestPDU(self.segmentAPDU.apduService)

            segAPDU.apduMaxSegs = encode_max_segments_accepted(self.maxSegmentsAccepted)
            segAPDU.apduMaxResp = encode_max_apdu_length_accepted(self.maxApduLengthAccepted)
            segAPDU.apduInvokeID = self.invokeID

            # segmented response accepted?
            segAPDU.apduSA = self.segmentationSupported in ('segmentedReceive', 'segmentedBoth')
            if _debug: SSM._debug("    - segmented response accepted: %r", segAPDU.apduSA)

        elif self.segmentAPDU.apduType == ComplexAckPDU.pduType:
            if _debug: SSM._debug("    - complex ack context")

            segAPDU = ComplexAckPDU(self.segmentAPDU.apduService, self.segmentAPDU.apduInvokeID)
        else:
            raise RuntimeError("invalid APDU type for segmentation context")

        # maintain the the user data reference
        segAPDU.pduUserData = self.segmentAPDU.pduUserData

        # make sure the destination is set
        segAPDU.pduDestination = self.pdu_address

        # segmented message?
        if (self.segmentCount != 1):
            segAPDU.apduSeg = True
            segAPDU.apduMor = (indx < (self.segmentCount - 1)) # more follows
            segAPDU.apduSeq = indx % 256                       # sequence number

            # first segment sends proposed window size, rest get actual
            if indx == 0:
                if _debug: SSM._debug("    - proposedWindowSize: %r", self.ssmSAP.proposedWindowSize)
                segAPDU.apduWin = self.ssmSAP.proposedWindowSize
            else:
                if _debug: SSM._debug("    - actualWindowSize: %r", self.actualWindowSize)
                segAPDU.apduWin = self.actualWindowSize
        else:
            segAPDU.apduSeg = False
            segAPDU.apduMor = False

        # add the content
        offset = indx * self.segmentSize
        segAPDU.put_data( self.segmentAPDU.pduData[offset:offset+self.segmentSize] )

        # success
        return segAPDU
This function returns an APDU coorisponding to a particular
        segment of a confirmed request or complex ack.  The segmentAPDU
        is the context.