A.G(S)/[1+G(S)H(S)] B.G(S)/[1-G(S)H(S)] C.G(S)/[1+H(S)] D.G(S)/[1-H(S)]
A.G1(S)+G2(S) B.G1(S)-G2(S) C.G1(S)G2(S) D.G1(S)/G2(S)
A.G1(S)+G2(S) B.TI-G2(S) C.G1(S)G2(S) D.G1(S)/G2(S)