The elbow can have {HEAD ~ two nestable channel sections}, each channel section having {TAIL ~ a generally C shaped cross section}, which may be substantially semi circular 
The elbow can have { HEAD ~ two nestable channel sections } , each channel section having { TAIL ~ a generally C shaped cross section } , which may be substantially semi circular
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH