Until recently I had no use for User Defined Aggregates, that was until I was ridiculed by my MySQL counterparts over the fact that Informix does not have a group_concat aggregate function.
This will not stand, ya know, this aggression will not stand, man.
A quick search of the internets produced this
Stack Overflow post which shows how to create a User Defined Aggregate in Informix that will duplicate the MySQL group_concat aggregate.