If the sentence does contain focus, negation yields a focus-associated reading (6a) and a non-associated reading (6b), as Kawamura (2007) illustrates with the example below.
this ambiguity has been regarded as a result of the scope interaction of focus and negation (Jackendoff, 1972;Krifka, 2006 we think  improved upon the task by scoring each and every potential positive interpretation generated from the negation.