code: private void addPoint(Point point, Collidable collidable)
      {
          final Integer group = collidable.getGroup();
          if (!collidables.containsKey(group))
          {
              collidables.put(group, new HashMap<Point, Set<Collidable>>());
          }
          final Map<Point, Set<Collidable>> elements = collidables.get(group);
          if (!elements.containsKey(point))
          {
              elements.put(point, new HashSet<Collidable>());
          }
          elements.get(point).add(collidable);
      }
docs: Add point. Create empty list of not existing.
  
  @param point The point to remove.
  @param collidable The associated collidable.