Function minimal_block_system 

pub fn minimal_block_system(
    degree: usize,
    gens: &[Perm],
) -> Option<Vec<Vec<usize>>>

The minimal non-trivial block system of a TRANSITIVE permutation group — the finest G-invariant partition into equal-size blocks bigger than a point and smaller than the whole set — or None if the group is primitive (only the trivial partitions are invariant) or not transitive. Imprimitivity is the symmetry’s internal structure: a grid symmetry decomposes into its rows, a cyclic group of composite order into cosets. (Atkinson’s algorithm over each pair {0, β}.)