I am new in Reduce, so I ask forgive me this question, it is may be simple.
I am trying to define rule for an operator

operator f1;
r1 := {f1(~x)+f1(~y)=> f(x+y)};

but when i try to use it

( f1(x) + f1(y) where r1 );

i have error message

***** Unmatched free variable(s) ~y

Is it possible to define rule for such operation ( not for f(~x+~y)=>f(x)+f(y), there isn't problem here)?

So please help me.

Best regards, Petrov Alexander.