Double Negative

A case for the Third Universal Cardinal Rule of Thumb: Never be absolute, unless absolutely necessary:

A linguistics professor was lecturing to his class one day. "In English," he said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a  negative. However, there is no language wherein a double positive can form a negative."

A voice from the back of the room piped up, "Yeah, right."