r*r^(k-1) -> r^k / Declaring variable ranges

Hello,
by default, axiom will simplify an expression like "r * r^5" into "r^6". 
However, an expression like "r * r^(k-1)" is untouched and not turned into 
"r^k". How can I change this?
  I guess depending on the type of k's values, the substitution cannot be done 
in general. But how do I tell axiom that while "k" is a "Variable k", /the 
value/ of any concrete "k" is, e.g., an Integer?
-- 
Gerald Knizia