From ComputerCraft Wiki
|Returns the running coroutine. If only the implicit top-level coroutine is running, nil is returned; however, ordinary applications will never see this because the CraftOS BIOS runs the shell, and therefore any application code, inside a coroutine.|
|Returns||coroutine the executing coroutine|
Please see Coroutine (API) for a worked example of using all the coroutine functions.