pub fn parse_gml<NodeWeightFn, EdgeWeightFn, N, E>( s: &str, node_weight_fn: &NodeWeightFn, edge_weight_fn: &EdgeWeightFn, ) -> Result<Graph<N, E, Directed>, &'static str>where NodeWeightFn: Fn(Option<&Sexp>) -> Option<N>, EdgeWeightFn: Fn(Option<&Sexp>) -> Option<E>,