in src/node_template.rs [320:325]
fn merge(lch: &MTreeNodeSmt<D>, rch: &MTreeNodeSmt<D>) -> MTreeNodeSmt<D> {
let mut hasher = D::new();
hasher.update(&lch.hash);
hasher.update(&rch.hash);
MTreeNodeSmt::new(hasher.finalize().to_vec())
}