my @idx = (
  [0, undef, 1, undef, undef, 2, 3, 4, 5, 6],  # X
  [0, 1, 2, 3, 4, 5, 6, undef, 7, 8],          # Y
  [0, undef, 1, 2, 3, 4, 5, undef, 6, 7],      # Z
);