Function reachable_end 

pub fn reachable_end(at: i64, until: Option<i64>, nights: &[(i64, i64)]) -> i64

When Reachable pressed at at ends at the latest (Unix seconds), whatever its press says, as every reader reads it (decision 8; review of part 3, F2): its own end (until; none: an older, a later or a garbled press), and never past the first night’s start after it when pressed by day, the waking of the night it was pressed in when pressed at night (nights, as Health keeps them), nor REACHABLE_MOST after it.