--- a/schema.py	Thu Nov 12 12:07:59 2009 +0100
+++ b/schema.py	Thu Nov 12 12:08:30 2009 +0100
@@ -806,6 +806,8 @@
                 mainvars.append('S')
             if 'O' in defined:
                 mainvars.append('O')
+            if 'U' in defined:
+                mainvars.append('U')
             if not mainvars:
                 raise Exception('unable to guess selection variables')
             mainvars = ','.join(mainvars)