My math class is so not PG. In talking about functions, we reference girls who are ho's (if an x - girl - has more than one y - guy - then, she does not have functional relations), and guys who have game (if a y - guy - has more than one x - girl - then it does not necessarily mean that the relation is not a function). And then you have the open circle which means that that point is not inclusive, or rather that the girl does not have a beau at that given point. 

Anyway.. kind of awkward. But I guess it works for them understanding the concept..