diff --git a/cpdfannot.ml b/cpdfannot.ml
index 9b04569..82f6335 100644
--- a/cpdfannot.ml
+++ b/cpdfannot.ml
@@ -1,3 +1,4 @@
+(** A loose JSON equivalent of XFDF. *)
 open Pdfutil
 open Cpdferror