Function subspace_polynomial 

pub fn subspace_polynomial<F: TowerLevel>(j: usize, x: F) -> F

The subspace polynomial W_j of S_j, defined by W_0(x) = x and W_j(x) = W_{j−1}(x)² + W_{j−1}(x).

W_j is F_2-linear, vanishes exactly on S_j, and satisfies W_j(v_j) = v_0 = 1, so it is already the normalised subspace polynomial Ŵ_j for the Cantor basis (D8).