Function expand_tail 

async fn expand_tail<'db>(
    env: &mut Env<'db>,
    live_after: LivePlaces,
    direction: Direction,
    unexpanded_linkvecs: Vec<Vec<RedLink<'db>>>,
    expanded_chains: Vec<RedChain<'db>>,
    consumer: &mut Consumer<'_, 'db, Vec<RedChain<'db>>, Errors<()>>,
) -> Errors<()>

After we’ve done the initial expansion to red links, we may end up with a chain that ends in something like mut[p] or ref[p]. In that case, we need to find the permissions from p and append them to the chain. This is called “expansion” in the dada-model code. This function expands the tail recursively until there are no more permissions to add and then invokes consumer.consume.