Function invoke 

pub fn invoke<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>,
    qn: &QName,
    a: &ActualParameters<N>,
    _ns: &NamespaceMap,
) -> Result<Sequence<N>, Error>

Invoke a callable component