Function chain_step 

pub fn chain_step(
    public_param: &PublicParam,
    epoch: u32,
    chain_index: usize,
    step: usize,
    x: &Digest,
) -> Digest

One chain step.

The position chain_index * CHAIN_LENGTH + step identifies the edge from chain value step to step + 1, so no two edges anywhere in the instance share a tweak.