diff --git a/combo/utils/graph.py b/combo/utils/graph.py
index 651c14a7d79b7ea3c277b9466f5e050435a7a01b..3352625e6665ca1cd3196506ed5e50183fedfbb0 100644
--- a/combo/utils/graph.py
+++ b/combo/utils/graph.py
@@ -88,6 +88,7 @@ def _dfs(graph, start, end):
 
 
 def restore_collapse_edges(tree_tokens):
+    # https://gist.github.com/hankcs/776e7d95c19e5ff5da8469fe4e9ab050
     empty_tokens = []
     for token in tree_tokens:
         deps = token["deps"].split("|")