DOC: Add code comments for StableGraph free lists
diff --git a/src/graph_impl/stable_graph/mod.rs b/src/graph_impl/stable_graph/mod.rs
index c407565..9a1d1f6 100644
--- a/src/graph_impl/stable_graph/mod.rs
+++ b/src/graph_impl/stable_graph/mod.rs
@@ -104,6 +104,14 @@
g: Graph<Option<N>, Option<E>, Ty, Ix>,
node_count: usize,
edge_count: usize,
+
+ // node and edge free lists (both work the same way)
+ //
+ // free_node, if not NodeIndex::end(), points to a node index
+ // that is vacant (after a deletion). The next item in the list is kept in
+ // that Node's Node.next[0] field. For Node, it's a node index stored
+ // in an EdgeIndex location, and the _into_edge()/_into_node() methods
+ // convert.
free_node: NodeIndex<Ix>,
free_edge: EdgeIndex<Ix>,
}
