For example, some organic type waste materials are now believed to pose {HEAD ~ possible valuable sources} of {TAIL ~ methane} as naturally occurring fossil fuels are depleted 
of
For example , some organic type waste materials are now believed to pose { HEAD ~ possible valuable sources } of { TAIL ~ methane } as naturally occurring fossil fuels are depleted
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH