Abstract: An elongated frame has a head section, a foot section and a center section. Hinges connect the center section with the head section and with the foot section, respectively. The head section is pivoted to a support in the region where it is connected to the center section, so it can tilt about a first axis which extends transversely of the elongation of the frame. Below the center section the support is provided with pivots which define a second axis extending parallel to the first axis; on these pivots, tiltable elements are pivotably mounted. Each of the tiltable elements is formed with a guide passage extending normal to the second axis and through which an elongated guide member mounted on a lower side of the center section extends in slidable relationship.