Function parse_nth 

pub fn parse_nth<'i>(
    input: &mut Parser<'i, '_>,
) -> Result<(i32, i32), BasicParseError<'i>>

Parse the An+B notation, as found in the :nth-child() selector. The input is typically the arguments of a function, in which case the caller needs to check if the arguments’ parser is exhausted. Return Ok((A, B)), or an Err(..) for a syntax error.