docs: private final void donCompletion( CountedCompleter caller ) {   // Distributed completion
      assert _lo == null || _lo.isDone();
      assert _hi == null || _hi.isDone();
      // Fold up results from left & right subtrees
      if( _lo    != null ) reduce2(_lo.get());
      if( _hi    != null ) reduce2(_hi.get());
      if( _local != null ) reduce2(_local   );
      // Note: in theory (valid semantics) we could push these "over the wire"
      // and block for them as we're blocking for the top-level initial split.
      // However, that would require sending "isDone" flags over the wire also.
      // MUCH simpler to just block for them all now, and send over the empty set
      // of not-yet-blocked things.
      if(_local != null && _local._fs != null )
        _local._fs.blockForPending(); // Block on all other pending tasks, also
      _keys = null;                   // Do not return _keys over wire
      if( _top_level ) postGlobal();
    }