code: public static Frame mmul(Frame x, Frame y) {
      MatrixMulJob mmj = new MatrixMulJob(Key.make("mmul" + ++cnt),Key.make("mmulProgress"),x,y);
      mmj.fork()._fjtask.join();
      DKV.remove(mmj._dstKey); // do not leave garbage in KV
      mmj._z.reloadVecs();
      return mmj._z;
    }
docs: to be invoked from R expression