Function ziv_round 

pub fn ziv_round<F>(p: usize, rm: RoundingMode, compute: F) -> ExactNum
where
    F: FnMut(usize) -> ExactNum,

Evaluate compute at increasing working precision until the result rounds uniquely to p bits (try_set_precision). Same retry budget as the transcendental kernel (crate::MAX_PREC_RETRY).