Using solve and sigma from programming

[photometer]

Hello,
I have an equation

pv = cf/(1+r) + cf/(1+r)^1 + cf/(1+r)^2 + ... + cf/(1+r)^n

I have the value of pv, cf and I need to find the value of r. How do I call the CAS.solve() and pass a function that does summation?

BEGIN
LOCAL pv,cf,result;
pv:= 100;
cf:= 10;
result:=CAS.solve(.....); // <- how do I write this?


END;