diff -r ddf4f19eb07a -r 8f63691ccb7f server/rqlannotation.py
--- a/server/rqlannotation.py	Fri Feb 26 13:21:13 2010 +0100
+++ b/server/rqlannotation.py	Fri Feb 26 13:24:47 2010 +0100
@@ -130,7 +130,8 @@
 
 
 
-class CantSelectPrincipal(Exception): pass
+class CantSelectPrincipal(Exception):
+    """raised when no 'principal' variable can be found"""
 
 def _select_principal(sqlscope, relations, _sort=lambda x:x):
     """given a list of rqlst relations, select one which will be used to