Function shells_within 

fn shells_within(
    dim: usize,
    radius: f64,
    dual: &[[f64; 3]; 3],
    reduction: Option<&Reduction>,
) -> i32

Cube shells of a sum over a lattice, reduced by reduction, whose dual rows are dual (a_i . b_j = 2 pi delta_ij): shells, or where reaching the lattice vectors within radius needs more, that many plus two, up to four times the base. Index i of a lattice vector x is c_i . x / 2 pi for the dual rows c of the reduced basis.