A second separating unit contains {TAIL ~ a stack} of {HEAD ~ insert material}, one insert at a time being separated and pushed forward 
A second separating unit contains { TAIL ~ a stack } of { HEAD ~ insert material } , one insert at a time being separated and pushed forward
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH