pub fn then_ref<RDF, P, F, N>(parser: P, function: F) -> ThenRef<P, F>where RDF: FocusRDF, P: RDFNodeParse<RDF>, F: FnMut(&P::Output) -> N, N: RDFNodeParse<RDF>,
Equivalent to p.then_ref(f).
p.then_ref(f)