theorem recneq2 (z: nat) (_S1 _S2: set) (n: nat): $ _S1 == _S2 -> recn z _S1 n = recn z _S2 n $;
_S1 == _S2 -> _S1 == _S2
_S1 == _S2 -> recn z _S1 n = recn z _S2 n