{HEAD ~ each} of {TAIL ~ X} is independently chosen from the moieties —N—, —C—, —O—, —OC—, —C—, —NHC—, and —NH—C— 
{ HEAD ~ each } of { TAIL ~ X } is independently chosen from the moieties — N — , — C — , — O — , — OC — , — C — , — NHC — , and — NH — C —
HEAD HEAD HEAD HEAD HEAD REL 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