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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 get_variable(self, variable_name, check_existence = False):
 """ Get a variable from the tax and benefit system. :param variable_name: Name of the requested variable. :param check_existence: If True, raise an error if the requested variable does not exist. """

        variables = self.variables
        found = variables.get(variable_name)
        if not found and check_existence:
            raise VariableNotFound(variable_name, self)
        return found