Function timeline_prev_batch 

fn timeline_prev_batch(
    timeline_pdus: &[(PduCount, PduEvent)],
    window_end: PduCount,
) -> Option<PduCount>

Returns the backward pagination token for a timeline slice.

A slice beginning at room creation has nothing before it, so its window’s end lets a members query describe the room as received. Backward pagination from that token re-reads the slice once before ending.