code: final CompletableFuture<T> postFire(CompletableFuture<?> a, int mode) {
          if (a != null && a.stack != null) {
              if (mode < 0 || a.result == null)
                  a.cleanStack();
              else
                  a.postComplete();
          }
          if (result != null && stack != null) {
              if (mode < 0)
                  return this;
              else
                  postComplete();
          }
          return null;
      }
docs: Post-processing by dependent after successful UniCompletion
  tryFire.  Tries to clean stack of source a, and then either runs
  postComplete or returns this to caller, depending on mode.