Fix zoom schema: content_url is optional

diff --git a/xml/schema/filter_zoom.rnc b/xml/schema/filter_zoom.rnc
index ece0c09..a9d75ec 100644 (file)
--- a/xml/schema/filter_zoom.rnc
+++ b/xml/schema/filter_zoom.rnc
@@ -8,7 +8,7 @@ filter_zoom =
   attribute name { xsd:NCName }?,
   element mp:torus {
     attribute url { xsd:string },
-    attribute content_url { xsd:string },
+    attribute content_url { xsd:string }?,
     attribute realm { xsd:string },
     attribute xsldir { xsd:string }?,
     attribute element_transform { xsd:string }?,