diff --git a/sexp.h b/sexp.h
index b751b0c278c9b48a81f23005beef0ed63d6149e3..3f7bcec432ea8c9bbaa686ac217e77f5b912b0dd 100644
--- a/sexp.h
+++ b/sexp.h
@@ -35,6 +35,9 @@ struct sexp_iterator
 {
   unsigned length;
   const uint8_t *buffer;
+  
+  /* If type is SEXP_LIST, pos points at the start of the current
+   * element. Otherwise, it points at the end. */
   unsigned pos;
   unsigned level;