extending in the longitudinal direction l as illustrated in fig6 b accordingly the hinge pins 54 can slide within the hinge pin accommodating holes 52 b provided at the upper forwardly extending lugs 52 b of elements 41 42 in longitudinal direction l on the other hand the lower forwardly extending lugs 52 of elements 41 and 42 comprise each a hinge pin accommodating bore similar to the upper and lower forwardly extending lugs 52 of elements 21 22 23 24 25 in which the hinge pins 54 are accommodated with a small play only sufficient to enable the hinge pins 54 to slightly incline within the hinge pin accommodating bores therefore an axis for a pivotal movement of elements 41 42 with respect to elements 25 41 respectively is provided at the interface of bottom ends of elements 41 25 and 41 42 which is substantially parallel to the width direction w moreover a pivotal movement of elements 41 42 with respect to elements 21 41 respectively is limited by the length of the elongated hinge pin accommodating hole 52 b provided in the upper forwardly extending lugs 52 b of elements 41 42 to a predetermined angle of pivotal movement a which is e g 1 to 10 degrees as can be seen from fig4 a and 4 b elements 41 and 42 are aligned with each other and with respect to element 25 in depth direction d so that their bottom ends 41 e 42 e are on substantially the same level with a bottom end of element 25 a supporting mechanism 60 for the strand like material m is attached e g bolted to the.