spec.setArgs("--provision", "--provider", "virtualbox");
        });
        isVMStarted = true;
    }

    // stops the VM if refs are down to 0, or force was called
    void maybeStopVM(boolean force) {
        assert refs >= 1;
        this.refs--;
        if ((refs == 0 || force) && isVMStarted) {
            execute(spec -> spec.setCommand("halt"));

            callIfVagrantTask(task, VagrantMachine::maybeStartVM);
        }

        @Override
        public void afterExecute(Task task, TaskState state) {
            callIfVagrantTask(task, service -> service.maybeStopVM(state.getFailure() != null));
        }
    }