OR rules

[Daniel Mahler]

Hi,

I would like to be able to write parallel rules with disjunctive semantics eg

%.tar : %.tar.Z
      unzip $<

%.tar : %.tar.gz
      gunzip $<

Which would only run gunzip if there was a .gz file 
and only unzip if there was a .Z file and either one
if both were present.
Is there a way to do this in make?
I know that :: rules allow many rules for the sam target, but treat them
conjuctively (ie all rules fire).
I would like a way to say fire a single rule that can successfully
bottom out.

thanks
Daniel