diff --git a/fs/tree.go b/fs/tree.go
index f3a5197..2671c46 100644
--- a/fs/tree.go
+++ b/fs/tree.go
@@ -25,6 +25,7 @@ type Tree struct {
 	statFn    func([]string, string) (os.FileInfo, error)
 	readdirFn func([]string, string) ([]os.DirEntry, error)
 	children  map[string]*Tree
+	Data any // user-attached state
 }
 
 // Option configures a Tree.