code: public final CountedCompleter<?> getRoot() {
      CountedCompleter<?> a = this, p;
      while ((p = a.completer) != null)
        a = p;
      return a;
    }
docs: Returns the root of the current computation; i.e., this task if it has no completer, else its
  completer's root.
  
  @return the root of the current computation