Logical Negation Puns

In my computer science course, my very old professor busts this out during a discussion of logic:

"I went to a logic conference once, and the lecturer said, 'it's interesting that a double negative gives you a positive, isn't it? I mean, a double positive doesn't give you a negative.'

Someone from the back of the room scoffed and went, 'yeah, yeah.'"

Half the class chuckled, half the class groaned.