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