docs: 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]