{HEAD ~ Another system} allows {TAIL ~ an operator} to vary the width of a mark from an initial dimension 
allows
{ HEAD ~ Another system } allows { TAIL ~ an operator } to vary the width of a mark from an initial dimension
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH