In step 512, it is determined whether {HEAD ~ the first receiving unit} has {TAIL ~ an unused tuner} 
has
In step 512 , it is determined whether { HEAD ~ the first receiving unit } has { TAIL ~ an unused tuner }
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL