In the roundup tracker, superseder is a Link type (not multilink as in
classic tracker). If you put in two link id's (1169513,2550964) you get
a traceback:

File ".../roundup/cgi/templating.py", line 886, in __getitem__
if int(self._nodeid) > 0:
ValueError: invalid literal for int() with base 10: '1169513,2550964'

This should be put inside a try/catch block to give a better message
to the user.

Maybe a message like: Property superseder is a Link and only supports 
one value."