in src/node_template.rs [98:104]
fn prove_padding_node(&self, idx: &TreeIndex, secret: &Secret) -> HashNodeSmt<D> {
let data = TreeIndex::serialize(&[*idx]);
let mut pre_image = D::new();
pre_image.update(secret.as_bytes());
pre_image.update(&data);
HashNodeSmt::new(pre_image.finalize().to_vec())
}