code: @Override
      public boolean cancel(boolean mayInterruptIfRunning) {
          if (!completed && !cancelled) {
              stop();
              cancelled = true;
              return true;
          }
          return false;
      }
docs: 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)