along the axis 9 in the embodiments depicted in fig5 to 8 this distance 21 varies along a portion of the folding axis 9 representing at least 10 of the length of this folding axis 9 and preferably at least 25 and in particular 100 of the length of the folding axis 9 the length of the folding axis 9 corresponds to the distance between the axial ends 15 and 16 in particular in the embodiments illustrated in fig5 to 8 the supports 2 have an axis of symmetry s substantially superimposable on the preferential folding axis 9 however the scope of the invention is not exceeded if in displacing the normal line 17 along the folding axis 9 it is seen that the distance 21 varies along this axis 9 differently from the distance 22 between the first intersection point 18 and the third intersection point 20 for the purposes of the invention the expression u201c varying non monotonously manner u201d is understood to mean that the respective distances 21 and 22 pass through at least one extreme position for example a maximum in particular as depicted in fig5 the normal line 17 shown passes through this maximum conversely as depicted in fig8 the respective distances 21 and 22 pass through a minimum the distances 21 and 22 can also pass through two or more extreme positions in particular according to the embodiments in fig5 and 8 the single extreme position is reached in a central zone of the folding axis 9 whereas in the embodiment in fig7 this.