// Copy a1 to a2 for index tuples that are valid for both arrays.
  static void copyArray(AbstractArray a1, AbstractArray a2) {

    Debug.assertTrue(a1.getRank() == a2.getRank());
    int[] subscripts = new int[a1.getRank()];
    int axis = 0;
    copySubArray(a1, a2, subscripts, axis);
  }