Writing a template which displays only relevant parents

[Dr Rainer Woitok]

Greetings,

part of my template to display a changeset is

   {if(parents,"\n<-{p1rev}:{p1node|short}{ifeq(p2rev,-1,'',',{p2rev}:{p2node|short}')}")}

and this works.   However, I want to improve  this output  in such a way
that trivial parents (immediately preceding revisions) are replaced with
an asterisk.  Something like (in a hypothetical programming language):

   if ( rev - 1 == p1rev )
   then "*"
   else "{p1rev}:{p1node|short}"

   if ( p2rev == -1 )
   then ""
   elif ( rev - 1 == p2rev )
   then ",*"
   else ",{p2rev}:{p2node|short}"

But I haven't yet found a way to do math  (yes, it IS "the supreme nost-
algia of our time" :-) in a template.  Or is there any other way to ach-
ieve what I want?

Any pointers welcome.

Sincerely,
  Rainer