Function decay_start 

fn decay_start(
    j1: i32,
    j2: i32,
    m1: i32,
    m2: i32,
    maximum: i32,
    stop: i32,
) -> i32

The lowest j in stop..=maximum down to which the downward recurrence of the row from maximum = j1 + j2 grows in magnitude at every step, so that the symbols decay upward from j on; stop if they do down to stop. The mirror of growth_end.