def is_untouchable(converter, variable):
    """Return whether or not `variable` is "untouchable".

    A variable is "untouchable" if it cannot (or should not) be
    abstracted away when simplifying the model.

    """
    timers = converter.special_variables.get('timers', set())
    timer_values = converter.special_variables.get('timer_values', set())
    if variable in timers | timer_values:
        return True
    else:
        return False
