ve the values 0 or 1 such that a+b = 0 or 1; ##STR2## where R is as defined for group I, c = d = 0 when ##STR3## and c and d may have the values 0, or 1 when ##STR4## when d = 0, Y is selected from hydrogen, ##STR5## and when d = 1, Y is selected from hydrogen, -R1, ##STR6## where R1 is an alkyl group as defined for R in group I above; ##STR7## where f and g may have the values 0, or 1 such that f