it reaches one of the sets "B"i (such an event is called a collision) where it reflects according to the law “the angle of incidence is equal to the angle of reflection” (if it reaches one of the sets formula_15 , formula_16, the trajectory is not defined after that moment). Such dynamical system is called semi-dispersing billiard. If the walls are strictly convex, then the billiard is called dispersing. The naming is motivated by observation that a locally parallel beam of trajectories disperse after a collision with strictly convex part of a wall, but remain locally parallel after a collision with a flat section of a wall.