"diphenyl-methylene, cyclopentylidene and cyclohexylidene. X.sup.1 is most preferably methylene, R.sup.28 as alkyl preferably contains 1 to 6 carbon atoms and is e.g. methyl, ethyl, propyl, butyl or hexyl; R.sup.28 as cycloalkyl is preferably cyclopentyl" . . . .