{HEAD ~ each L} is {TAIL ~ a single covalent bond}, or L is a covalent linkage having 1 24 nonhydrogen atoms selected from the group consisting of C, N, O and S and is composed of any combination of single, double, triple or aromatic carbon carbon bonds, carbon nitrogen bonds, nitrogen nitrogen bonds, carbon oxygen bonds and carbon sulfur bonds; and 
{ HEAD ~ each L } is { TAIL ~ a single covalent bond } , or L is a covalent linkage having 1 24 nonhydrogen atoms selected from the group consisting of C , N , O and S and is composed of any combination of single , double , triple or aromatic carbon carbon bonds , carbon nitrogen bonds , nitrogen nitrogen bonds , carbon oxygen bonds and carbon sulfur bonds ; and
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH