Template Conditional Logic?

[Luke <..hidden..>]

Is there any way to implement conditional logic in templates, beyond 
variable set/unset if statements?

In particular, for cases where the duedate and invdate are the same in a 
tex invoice, I would like to print "UPON RECEIPT", instead of the 
actual due date.

Is there any convenient way to do this?

I have thought of at least one inconvenient way, using custom PERL and an 
alternate variable for duedate storage, but I'd like to avoid that if 
possible.

Luke