Function get_leaf_proof_with_leaf_chain 

pub(crate) async fn get_leaf_proof_with_leaf_chain<State>(
    state: &State,
    requested_leaf: LeafQueryData<SeqTypes>,
    fetch_timeout: Duration,
    chain_limit: usize,
) -> Result<LeafProof, Error>
where
    State: AvailabilityDataSource<SeqTypes> + VersionedDataSource + Clone + Send + Sync + 'static,
    for<'a> State::ReadOnly<'a>: NodeStorage<SeqTypes>,

Build a leaf proof for a legacy leaf from the justify QCs of the leaves that follow it.