Function merge_frontier_head 

pub fn merge_frontier_head<H>(
    frontier: &mut Vec<H>,
    head: H,
    stream_key: impl Fn(&H) -> CircleAuthorStreamKey,
    seq: impl Fn(&H) -> u64,
)

Insert head into a frontier keyed by author stream, keeping the deeper (higher-sequence) head when the stream already carries one. Merging every conflicting branch’s heads this way yields the union frontier: each stream is covered at its deepest position across all branches, so a device that authored on that stream continues from its own head instead of re-allocating it.