Abstract_meet tells whether a position is above the floor or below (i.e. overlap). It also gives the shortest vector from floor. Such vector will be useful to calculate the potential between a particle and the wall. Abstract_visit visits 2 Abstract_Floor_Penetration : one below (floor) and one above (ceiling) which are mirror symmetric with respect to the \( (x, y, 0) \) plan. The name may be confusing: walls are horizontal not vertical.