. . . "3 -aryl group or C6 -C10 --, preferably C6 --C8 -aryl group. ##STR4## in which R12 and R13 are identical or different and are a hydrogen atom, a halogen atom, a C1 -C10 --, preferably C1 -C4 -alkyl group, in particular a methyl group, a C1 -C10 -fluoroalkyl group, preferably a CF3 group, a C6 -C10 --, preferably C6 -C8 -aryl group, a C6 -C10 -fluoroaryl group, preferably a pentafluorophenyl group," . . . .