Function pop_first_eligible_or_first 

pub fn pop_first_eligible_or_first<M: Memory + Copy>(
    m: M,
    queue: u64,
    cutoff: u64,
    hold: bool,
) -> u64

Remove and return the first task whose eligibility is at or before cutoff, or else the first task, or 0 if the queue is empty. With hold, the task is held for the caller.