The coupling parts 28 29 are preferably also realized in {HEAD ~ the shape} of {TAIL ~ a tongue} 31 and a groove 32 
The coupling parts 28 29 are preferably also realized in { HEAD ~ the shape } of { TAIL ~ a tongue } 31 and a groove 32
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH