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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 focus_cb(self, viewer, channel):
 """ Callback from the reference viewer shell when the focus changes between channels. """

        chname = channel.name

        if self.active != chname:
            # focus has shifted to a different channel than our idea
            # of the active one
            self.active = chname
            self.set_info("Focus is now in channel '%s'" % (
                self.active))
        return True