Function unsigned 

pub fn unsigned<'a, N, F, P, I, C>(parser: F) -> impl Parser<I, Output = N> + 'a
where
    F: FnOnce(bool) -> P,
    P: Parser<I, Output = N> + 'a,
    I: Positioned<Ok = C> + ?Sized,
    C: Character,

Takes a function returns a integer parser, returns a parser of unsigned, positive integer.

This function is for symmetry with the function signed, so like it, the taken function must return negative result if the argument is true, and vice versa.