function
(propagate x fiber)
Propagate a signal from a fiber to the current fiber and set the
last value of the current fiber to `x`. The signal value is then
available as the status of the current fiber. The resulting stack
trace from the current fiber will include frames from fiber. If
fiber is in a state that can be resumed, resuming the current fiber
will first resume `fiber`. This function can be used to re-raise an
error without losing the original stack trace.