Trauma creeps up on you when you are distracted. Someone bends down to pick up a piece of paper that fell on the floor of the MTR train carriage, and even if your eyes are on your phone, you catch that movement with your peripheral vision, and your heart skips a beat. The images come back unsolicited, and play in the inscrutable theatre of one’s brain: people begging to be spared, kneeling in that same metallic, silvery doorway, crying for help and mercy. Police officers don’t care: they storm the train, they beat people they suspect of being protesters with...