[Nauty] nauty colouring question

[Joost Winne]

Say I have a graph with n = m*k vertices.
I want to allow permutations inside any group of k vertices with labels
(j*k, j*k + 1, ..., (j+1)*k - 1) for j in 0 <= j < m.

This would be easy by just setting lab[i] = i (0 <= i < n)
and ptn[i] = 0 only if (i % k == k - 1).

But what if I would like to allow permutations of one k-pack to another
k-pack as well ?

Say k = 3 and m = 2,
then
------
0 -> 4
1 -> 3
2 -> 5
------
3 -> 0
4 -> 1
5 -> 2
------
should be tested if it was an automorphism,
but (0 3) should not be tested.

Is there a clever way to specify this to nauty or do I have to specify no
initial colouring and then just look which automorphisms behave like the
way I am interested in.