The approach of ~ [1970, 1973, 1973a] has received most of the attention in the literature (Burmeister [1974], Faber [1975], Fehl [1975], ~[1975], Hagemann and ~ [1976]). The latter concept has been revived by the second group, to which Tintner [1970], von Weizs~cker [1971a, 1971b, 1974], ~ [1971, 1976 and ~ [1973, 1975, 1976] belong.