All that's useful for coroutines.
| Semaphore | Simple coroutine synchronization device. |
| beginCritical | Signals the VM that this coroutine must not be interrupted. |
| endCritical | Signals the VM that this coroutine can be interrupted. |
| sleep | Put the current coroutine at sleep for some time. |
| yield | gives up the rest of the coroutine time slice. |
| yieldOut | Terminates current coroutine. |