Preferably, the first housing has {HEAD ~ a third engagement part} to be engaged with {TAIL ~ the first mating part} 
to be engaged with
Preferably , the first housing has { HEAD ~ a third engagement part } to be engaged with { TAIL ~ the first mating part }
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL