code: public void complete(Void mustBeNull) {
          CountedCompleter p;
          onCompletion(this);
          quietlyComplete();
          if ((p = completer) != null)
              p.tryComplete();
      }
docs: Regardless of pending count, invokes {@link #onCompletion},
  marks this task as complete with a {@code null} return value,
  and further triggers {@link #tryComplete} on this task's
  completer, if one exists. This method may be useful when
  forcing completion as soon as any one (versus all) of several
  subtask results are obtained.
  
  @param mustBeNull the {@code null} completion value