@Override
    public boolean cancel(boolean mayInterruptIfRunning) {
        if (!completed && !cancelled) {
            stop();
            cancelled = true;
            return true;
        }
        return false;
    }
@ Override public boolean cancel ( boolean mayInterruptIfRunning ) { if ( ! completed && ! cancelled ) { stop ( ) ; cancelled = true ; return true ; } return false ; }
Prevents the invocation of further handlers (like {@link #stop()}
and (in addition) the invocation of any added completed events.

@param mayInterruptIfRunning ignored
@return `false` if the event has already been completed
@see java.util.concurrent.Future#cancel(boolean)