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

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]