Even for the Lincoln Centre it was an unusual show, and an unscheduled one. Several hundred protesters turned up outside the arts complex on Manhattan's Upper West Side last week for the guerrilla screening of a short film. From a hotel on the other side of the street, a video was projected on to the centre's walls. The unwitting stars of the films were David and Charles Koch, the reclusive rightwing billionaire brothers whose secretive empire and network of influence and funding is emerging as a liberal rallying cause in America.