Function grad_masked_adj 

fn grad_masked_adj(
    r: &[Complex64],
    mask_of: &[u32],
    gw: &[Vec<f64>],
    n: [usize; 3],
    bx: &Box3,
    acc: &mut [Complex64],
)

Adjoint of grad_masked onto the box bx of the grid: acc[l] = Σ_a (q_a[l − e_a] − q_a[l]) for l in bx (the rest of acc is left as it is), summed in axis order from zero, with q_a = gw[a]·r_a on the mask and zero elsewhere (mask_of[l] = position of l in inside, or NONE). The result is zero outside the mask grown by one voxel along +e_a.