diff --git a/projekte/protothreads/example.c b/projekte/protothreads/example.c
index 6334bb3..82844a4 100644
--- a/projekte/protothreads/example.c
+++ b/projekte/protothreads/example.c
@@ -1,3 +1,5 @@
+/* based on example-small.c in the protothreads tarball */
+
 #include "avr/io.h"
 #include "util/delay.h"