In operation, {HEAD ~ customers} enter into {TAIL ~ waiting line} 16 thus activating the entrance detector 18 
enter into
In operation , { HEAD ~ customers } enter into { TAIL ~ waiting line } 16 thus activating the entrance detector 18
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH