Question: algsubs with subscripts and powers

When attempting to use algsubs with a subscripted expression involving powers I am running into a problem.

I have assigned a numerical value to n, and I have a symbol subscripted with n+1 and raised to a power.

In the original expression I have a factor of the form: P^q + R_n^q which I should like to replace with the symbol B.

> n:= 2

> P^q + R_(n+1)^q

> algsubs(P^q + R_(n+1)^q = B, %)

This last input leads to the error message:

Error, (in algsubs) no variables appearing rationally in pattern.

If I attempt to use algsubs on a subscripted symbol which is not raised to a power, the result works. Is there a cure?

Thanks in advance.

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