How one writes circular circuit, for example stream of fibonnachi numbers or IIR filter? For IIR filter it would be nice if it has protoype like iir(sig : T, a

How one writes circular circuit, for example stream of fibonnachi numbers or IIR filter? For IIR filter it would be nice if it has protoype like iir(sig : T, a : vec<T, N>, b : vec<T,M>) -> T