of stationary Gaussian processes with mean zero, i.e., Si = {(Xt)tz; (Xt)t stationary Gaussian process with E[Xt] = O}. The second type is the set of genuine MA processes, i.e., S2 = {(Xt)tEz; Xt as defined in Eq. 1.1}. The third type which arises is more surprising. ...