Version 0 of Coroutine cancellability

You can't generally cancel a running coroutine easily. You can of course do

rename runningCoro ""

But then:

• the coroutine doesn't know it was cancelled(*), no error is raised inside it, it just ceases to exist
• if the coroutine has scheduled itself as a callback like in e.g.

::chan event $chan readable [list [info coroutine]]

just like say coroutine::util gets does, then that event will produce an error. Unfortunately I found no way to handle coroutine cancellation without resorting to callbacks. The good news is one can do

proc cleanup {args} {...}
proc coro {} {
    ...
    trace add command [info coroutine] delete [list cleanup $arg1 $arg2]
    ...
}

Which will allow it to e.g. unsubscribe from fileevent. Quite awkward: another proc has to be created and it also has too accept mostly args that trace passes to it in addition to any useful args we may pass from inside the coro.