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