<p>A first U-shaped element (FUSE) (3) bends at both ends (4) as a rectangle so that the ends have a clearance from a U-shaped surface. Two FUSEs link at their ends to an extendable frame (2) and the FUSE's ends interconnect through a reinforcing strut (5). A second element (SE) has an H-shape and a third element (7) a U-shape. The ends of the SE link to the FUSE's ends.</p>
