Function merkle_node 

pub fn merkle_node(
    public_param: &PublicParam,
    level: usize,
    index: u32,
    left: &Digest,
    right: &Digest,
) -> Digest

A Merkle parent, hashed under the tweak that fixes its level and index.

The level and index place each node in its own hash domain, so a node computed at one position cannot be replayed at another.