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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 _AddVariable(self, variable):
 """ Add a variable to the model. Should not be used by end-user """

        if isinstance(variable, Signal):
            if not variable in self.signals:
                self.signals.append(variable)
        elif isinstance(variable, Variable):
            if not variable in self.variables:
                self.variables.append(variable)
        else:
            raise TypeError
        self._utd_graph = False