sums in defrule (partitions sum)

Hi,

If I want to define a rule to rewrite an expression
which contains a sum like in this examples:

defrule(myr1, f(A+B), f(A)+g(B));
defrule(myr1, sin(A+B)*sin(A-B), cos(A)^2-cos(C)^2);

I get the warning:
B + (- A 1) partitions `sum'

and applying the rule does not work.

What is the problem with rules like that?
How could those rewriting rules be defined in maxima?

gr.
Anton