Preferred examples of A in formula (I) are lower alkyl groups having 1 to 3 carbon atoms such as a methyl group, and a phenyl or benzyl group with is unsubstituted or substituted by a substituent selectedfrom the class consisting of halogen atoms such as Cl or F, a trifluoromethyl group and lower alkyl groups having 1 to 3 carbon atoms such as a methyl group.The compounds of formula (I) of this in