inv_f1 = array1d(1..4, [5, 7, 6, 8]);
inv_f2 = array1d(5..8, [1, 3, 2, 4]);
----------
inv_f1 = array1d(1..4, [5, 7, 8, 6]);
inv_f2 = array1d(5..8, [1, 4, 2, 3]);
----------
==========