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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 on_default_shell_changed(self, combo):
 """Changes the activity of default_shell in dconf """

        citer = combo.get_active_iter()
        if not citer:
            return
        shell = combo.get_model().get_value(citer, 0)
        # we unset the value (restore to default) when user chooses to use
        # user shell as guake shell interpreter.
        if shell == USER_SHELL_VALUE:
            self.settings.general.reset('default-shell')
        else:
            self.settings.general.set_string('default-shell', shell)