S={single valued functions}
For a given g(x), if both p(x) and q(x) single valued and satisfying p(p(x))=g(x) and q(q(x))= g(x)
then pRq is an equivalent relation p~q and all such functions form an equivalent class.
證明：
1. 自反性： 顯然， pRp
2. 對稱性： pRq=qRp
3. 傳遞性： pRq and qRt then pRt
