A check is made whether {TAIL ~ a value} has been assigned to {HEAD ~ a particular alternate dispatch variable} which has been sequenced 
A check is made whether { TAIL ~ a value } has been assigned to { HEAD ~ a particular alternate dispatch variable } which has been sequenced
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH