An instant messaging service interacts with IM clients operating on computing devices to implement an extensible platform with which individual composer applications may interact to support a variety of different messaging user experiences for users of respective local and remote devices. The concrete slab has openings extending therethrough. The coupling means 23, 24 is designed to allow the second bracket 21, 22 to have an axial relative displacement with respect to the first bracket 20 on the basis of an axial direction of the steering shaft 9 while allowing an upward relative displacement of the second bracket 21, 22 with respect to the first bracket 20 in the course of the axial relative displacement, during a vehicle collision.