{TAIL ~ X} represents {HEAD ~ a number} from 2 to 16, or a physiologically acceptable salt thereof 
{ TAIL ~ X } represents { HEAD ~ a number } from 2 to 16 , or a physiologically acceptable salt thereof
TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH