Function event_thread_kind 

fn event_thread_kind(event: &ReceiptEvent) -> &str

Tag string used in the storage key to discriminate receipts per thread. Empty for Unthreaded, "main" for Main, the event-id string for Thread(...) (event ids start with $, so the values are mutually exclusive). Custom variants reuse their string form; the C/S boundary rejects them, but federation receipts may still carry them through.

Reads only the first (event_id, type, user) triple. All callers build single-entry receipts (one event id, one type, one user); a debug assertion catches future regressions. An entirely empty event or one whose only receipt lacks a thread field falls back to "".

Appended to the receipt-row key as a tolerant trailing field. Pre- MSC3771 rows have no trailing kind; they round-trip as "".