Find all f: R -> R, f not identically zero, such that

(*) f( (x+y)/(x-y) ) = ( f(x)+f(y) )/( f(x)-f(y) ).

Find all f: R -> R, f not identically zero, such that

(*) f( (x+y)/(x-y) ) = ( f(x)+f(y) )/( f(x)-f(y) ).

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