Function expr 

pub fn expr<'a, N: Node + 'a, L>() -> Box<dyn Fn(ParseInput<'a, N>, &mut StaticState<L>) -> Result<(ParseInput<'a, N>, Transform<N>), ParseError> + 'a>
where
    L: FnMut(&NamespacePrefix) -> Result<NamespaceUri, ParseError> + 'a,