help with rules for system table

Hi,

  I have to archieve functions like this:
  When users define a new index, I will do something (for instance,
increase an counter in my table or do some other statistics). However,
I defined rule for insert on pg_class and when the entry is actually an
index, I do my thing.  Obviously my rule is never executed. If I create
a similar rule for a table I defined, it works.  Does this mean that I
can't create rules for system tables?  If not, does anyone know how to
do this?

Any suggestions or solutions are appreciated.

Thanks,
Ruey-Lung Hsiao