def METHOD_NAME():
    class InstrumentC(Instrument):
        pass
    assert len(InstrumentC.get_instances()) == 0, "An instance of a not-yet-created instrument existed"
    c = InstrumentC()
    assert len(InstrumentC.get_instances()) == 1, "The created instrument wasn't in the list"
    del c
    assert len(InstrumentC.get_instances()) == 0, "The instrument wasn't properly removed on deletion"