Function write 

pub async fn write<S: NamespaceStore>(
    store: &S,
    source: &Partition,
    repo: &RepoName,
    pack: &Hash,
    prior: Option<&Value>,
    state: &VerificationV1,
    deadline_ms: u64,
) -> Result<bool, StoreError>

Three-operation state CAS: NotAfter, prior-value guard, and put.