code: final CompletableFuture<T> postFire(CompletableFuture<?> a, CompletableFuture<?> b, int mode) {
      if (b != null && b.stack != null) { // clean second source
        Object r;
        if ((r = b.result) == null)
          b.cleanStack();
        if (mode >= 0 && (r != null || b.result != null))
          b.postComplete();
      }
      return postFire(a, mode);
    }
docs: Post-processing after successful BiCompletion tryFire.