class['a]signal_engine :Unixqueue.event_system ->object..end
let se = new signal_engine esys: The engine se remains in
`Working 0 until the method se # signal x is called. At this point
e transitions to x. Any further call of signal does not
have any effect.
Also, if se is aborted, signal does not have any effect.
The function signal may be called from a different thread.
The signalling event is forwarded to the thread running the
event system.
method signal : 'a final_state -> unit
