. . . "A1 is C2 -C4 -alkylene, such as --CH2 CH2 --, --CH2 CH2 CH2 -- and --CH2 CH2 CH2 CH2 --, which may carry from one to three C1 -C3 -alkyl groups, such as methyl, ethyl, propyl and 1-methylethyl, in particular methyl;" .