stoppable.add(a)
        stoppable.add(b)

        when:
        stoppable.stop()

        then:
        1 * a.stop()
        1 * b.stop()
    }

    def stopsAllElementsWhenOneFailsToStop() {
        Stoppable a = Mock()
        Stoppable b = Mock()
        RuntimeException failure = new RuntimeException()
        stoppable.add(a)
        stoppable.add(b)

        when: