{"ast_node_type":"other","tokens":[{"end":88,"kind":"keyword","start":0,"text":"&mut"}]}
                &mut edge_pairs.iter().map(|&(_, tgt)| N::new(tgt.index() + num_nodes)),
                // NB: we double the key index, so that we can later use *2 to get the back references
                &mut edge_pairs.iter().map(|&(src, _)| src),