Function tier_level_by 

pub fn tier_level_by(
    n: usize,
    index: usize,
    smaller_is_urgent: bool,
    priority_of: impl Fn(usize) -> i64,
) -> Option<u8>

The level of tier index of an image whose n tiers have the kernel priorities priority_of(0..n): its urgency order, 0 the most urgent, equal priorities in array order. Until phase-509 P9 a level is a tier, so this is how a runner places each tier’s executor (Executor::set_level). nros codegen entry writes NROS_ENTRY_LEVEL_OF_<node> by the same rule, over the same array. None when index is out of range.