These units may also have other pieces such as microphones and speakers in order to allow {HEAD ~ communication} to be facilitated in {TAIL ~ difficult circumstances} 
to be facilitated in
These units may also have other pieces such as microphones and speakers in order to allow { HEAD ~ communication } to be facilitated in { TAIL ~ difficult circumstances }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL