If inside a side-effect, adds an effect to it.

def add_effect(effect_id, *args, **kwargs):
    '''If inside a side-effect, adds an effect to it.'''
    effect = fiber.get_stack_var(SIDE_EFFECT_TAG)
    if effect is None:
        return False
    effect.add_effect(effect_id, *args, **kwargs)
    return True