Function plan_delete 

pub fn plan_delete(
    current: Option<&Record>,
    expected: Option<Revision>,
    now_ms: i64,
    cap: usize,
    author: Option<&Identity>,
) -> DeletePlan

Decide a conditional delete (spec §3.2). Deleting an absent key writes nothing: there is no revision to order a tombstone against a peer’s copy. Deleting a tombstone writes nothing: the chain does not advance.