Function sign 

pub fn sign<'a, I, C>(plus_sign: bool) -> impl Parser<I, Output = bool> + 'a
where
    I: Input<Ok = C> + ?Sized + 'a,
    C: Character + 'a,

Parses a plus or minus sign and returns true if it is a minus sign.

By the default, plus signs are not allowed and it will be allowed if the argument plus_sign is true.