In universal algebra, a clone is a set C of finitary operations on a set A such that C contains all the projections πkn: An → A, defined by πkn = xk, C is closed under composition : if f, g1, …, gm are members of C such that f is m-ary, and gj is n-ary for all j, then the n-ary operation h := f is in C.The question whether clones should contain nullary operations or not is not treated uniformly in the literature. Wikipedia