Just how tough is NLP?

I got this while auditing an introductory class on natural language processing.

A colleague recently told Roger Gould, a sociologist at the University of Chicago, about a lecture, place uncertain, referring to double negatives. Every language, the lecturer observed, has a construction in which two negatives make a positive. But in English, he said, there's no construction in which two positives make a negative.

From the hall came the perfect, anonymous response: "Yeah, right."