{HEAD ~ The print head unit} includes {TAIL ~ a first print head} , a second print head , a third print head , and a print head frame  
{ HEAD ~ The print head unit } includes { TAIL ~ a first print head } , a second print head , a third print head , and a print head frame
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH