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.
Inherits
method signal : 'a final_state -> unit
