. . "rom the group consisting of an alkylene group including a substituted alkylene group, an arylene group including a substituted arylene group; S is sulphur, and B is a member selected from the group consisting of an alkyl group including a substituted alkyl group, and an aryl group including a substituted aryl group as disclosed e.g. in U.S. Pat. No. 4,546,059 such as : ##STR6## and 4-dicyanomethyl" . .