Function immutable_seq::digit::split_once

pub fn split_once<'a, T, M, P>(pred: &P,
                               i: M,
                               digit: &'a Digit<T, M>)
                               -> (Option<Digit<T, M>>, &'a Lazy<Node<T, M>>, Option<Digit<T, M>>) where T: Measure<M> + 'static,
        M: Add<Output=M> + Copy + 'static,
        P: Fn(M) -> bool