In 1970, Fang Zhongmou, a doctor in the eastern Chinese province of Anhui, was charged as a counter-revolutionary and executed. What makes the case unusual is that she was killed after being denounced by her teenage son, Zhang Hongbing. Zhang, 60, describes his role in his mother's death, which he now bitterly regrets, but is publicising to highlight the 'catastrophe' of the 1966-76 Chinese Cultural Revolution