p ##STR3## a group --NR.sup.2 -- or a group --N.dbd.; where R.sup.2 is a hydrogen atom, a hydrocarbyl group, asubstituted hydrocarbyl group or two groups R.sup.2, together with the carbon atom, or carbon atoms, to which they are attached form a ring.The groups A, B and D can form part of a further ring system but generally not more than two of the groups A, B and D form part of a further ring syst