A compound as claimed in claim 1, in whichA is alkylene of 2 to 6 carbon atoms, or phenylene unsubstituted or substituted by 1 or 2 substituents selected from the group consisting of alkoxy of 1 to 4 carbon atoms, alkyl of 1 to 4 carbon atoms, chlorine, sulfo and carboxy, or A is a group of the formula ##STR90## in which W is a direct bond or a bridge member selected from the group consisting of t