diff --git a/module/srfi/srfi-13.mes b/module/srfi/srfi-13.mes
index 0bd8c543..a801d7db 100644
--- a/module/srfi/srfi-13.mes
+++ b/module/srfi/srfi-13.mes
@@ -1,7 +1,7 @@
 ;;; -*-scheme-*-
 
 ;;; Mes --- Maxwell Equations of Software
-;;; Copyright © 2016,2017 Jan Nieuwenhuizen <janneke@gnu.org>
+;;; Copyright © 2016,2017,2018 Jan Nieuwenhuizen <janneke@gnu.org>
 ;;;
 ;;; This file is part of Mes.
 ;;;
@@ -90,3 +90,5 @@
 
 (define substring/copy substring)
 (define substring/shared substring)
+
+(define string-null? (compose null? string->list))