code: public boolean cancel(boolean mayInterruptIfRunning) {
      boolean cancelled =
          (result == null) && internalComplete(new AltResult(new CancellationException()));
      postComplete();
      return cancelled || isCancelled();
    }
docs: If not already completed, completes this CompletableFuture with a
  {@link CancellationException}. Dependent CompletableFutures that have not already completed
  will also complete exceptionally, with a {@link CompletionException} caused by this
  {@code CancellationException}.
  
  @param mayInterruptIfRunning this value has no effect in this implementation because interrupts
  are not used to control processing.
  
  @return {@code true} if this task is now cancelled