QUES: unacceptable
ANS: The cat that the dog that the rat bit chased died.

QUES: unacceptable
ANS: Maxwell isn't half the doctor that polished off the vodka.

QUES: acceptable
ANS: Joe will go to the store, even though Fred already has.

QUES: acceptable
ANS:
Gilgamesh might not have been reading the cuneiform tablets.