The sampler 23 wherein {HEAD ~ the wall} comprises an inner wall and {TAIL ~ an outer wall}, said walls being concentric with each other 
comprises
The sampler 23 wherein { HEAD ~ the wall } comprises an inner wall and { TAIL ~ an outer wall } , said walls being concentric with each other
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH