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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_phi_subvariables(self, var):
 """ Get sub-variables that phi variable `var` represents. :param SimVariable var: The variable instance. :return: A set of sub-variables, or an empty set if `var` is not a phi variable. :rtype: set """


        if not self.is_phi_variable(var):
            return set()
        return self._phi_variables[var]