The children of Demon Veen and Marina Durmanov, Van and Ada (the two main characters in VN’s novel Ada, 1969) are officially maternal cousins and can marry only by special decree:
They walked through a grove and past a grotto.
Ada said: ‘Officially we are maternal cousins, and cousins can marry by special decree, if they promise to sterilize their first five children. But, moreover, the father-in-law of my mother was the brother of your grandfather. Right?’