But {HEAD ~ water} may pool at {TAIL ~ the top end} of the array, and on structures such as rooftops, the pooling may have undesirable consequences 
pool at
But { HEAD ~ water } may pool at { TAIL ~ the top end } of the array , and on structures such as rooftops , the pooling may have undesirable consequences
OTH HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH