Function format_number 

pub fn format_number<N: Node, F: FnMut(&str) -> Result<(), Error>, G: FnMut(&str) -> Result<N, Error>, H: FnMut(&Url) -> Result<String, Error>>(
    ctxt: &Context<N>,
    stctxt: &mut StaticContext<N, F, G, H>,
    num: &Transform<N>,
    picture: &Transform<N>,
    _name: &Option<Box<Transform<N>>>,
) -> Result<Sequence<N>, Error>

XPath format-number function.