code: private void removePoint(Point point, Collidable collidable, Map<Point, Set<Collidable>> elements, Integer group)
      {
          final Set<Collidable> points = elements.get(point);
          points.remove(collidable);
          if (points.isEmpty())
          {
              elements.remove(point);
          }
          if (elements.isEmpty())
          {
              collidables.remove(group);
          }
      }
docs: Remove point. Remove list of no more collidable.
  
  @param point The point to remove.
  @param collidable The associated collidable.
  @param elements The current group elements.
  @param group The current group.