A distinguished linguistics professor was lecturing on the phenomenon of
double negatives. As he neared the end of his talk, he drew himself up and
declared solemnly:
In conclusion, let me observe that while there are numerous cases where a
double negative conveys a positve, there is no case where a double positive
conveys a negative.
Whereupon, from the back of the room, arose a small voice dripping with
disdainful condescension:
Yeah, yeah…