{HEAD ~ A retainer} extends about {TAIL ~ the periphery} of the skylight for holding the panels on the frame means 
extends about
{ HEAD ~ A retainer } extends about { TAIL ~ the periphery } of the skylight for holding the panels on the frame means
HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH