Double negatives

[baulch@xxxxxxxxxxxxxxxxxxxxx (Garth Baulch)]

A linguistics professor was lecturing to his class one day about the
fact that in many languages, such as English, a double negative forms a
positive, while in other languages, such as Russian, a double negative
is still a negative.  "However," he pointed out, "in no language can a
double positive form a negative."

A bored voice from the back of the room responded, "Yeah, yeah...."

[Ed:  Reportedly a true incident, accounted in the New Yorker Magazine.
]