diff --git a/content/concurrency.article b/content/concurrency.article
index 9bb55c0..bdde17d 100644
--- a/content/concurrency.article
+++ b/content/concurrency.article
@@ -128,6 +128,8 @@
 
 `Same(tree.New(1),`tree.New(1))` should return true, and `Same(tree.New(1),`tree.New(2))` should return false.
 
+The documentation for `Tree` can be found [[https://godoc.org/golang.org/x/tour/tree#Tree][here]].
+
 .play concurrency/exercise-equivalent-binary-trees.go
 
 * sync.Mutex