Type Alias Frontier 

pub type Frontier = BTreeMap<String, u64>;

A pull cursor: device_id → highest seq the puller already holds for that device. Seq-based (not HLC-based) on purpose: a straggler’s late ops carry old HLCs but new seqs, so a seq frontier still delivers them to every peer — an HLC cursor would silently skip them.

Aliased Type

pub struct Frontier { /* private fields */ }