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