There are two interesting points in his approach: First, Chomsky connects these syntactic issues with the placement of the intonation center in the spoken form of the sentence, and second, he connects the possible operational criterion for the determination of the choice of focus from the range of permissible focus with the scope of negation.
to decide what is the focus of the answer to Was it an ex-convict with a red SHIRT that he was warned to look out for?, one should consider possible different negative continuations such as No, he was warned to look out for an AUTOMOBILE SALESMAN, or .