diff --git a/yslt.yml2 b/yslt.yml2
index 338ede1..f5caf3c 100644
--- a/yslt.yml2
+++ b/yslt.yml2
@@ -39,6 +39,7 @@ in xsl {
         xmlns:str='http://exslt.org/strings',
         xmlns:dyn='http://exslt.org/dynamic',
         xmlns:set='http://exslt.org/sets',
+        xmlns:sets='http://exslt.org/sets',
         xmlns:date='http://exslt.org/dates-and-times',
         xmlns:yml='http://fdik.org/yml',
         extension-element-prefixes='exsl func str dyn set math date yml'