Section 2.1 (FN) Vertical Line Test Definitioon

Definition 2.1.17 feels a little awkward to me. I would suggest something like: The **vertical line test** is a method to determine whether a relation is a function by checking whether vertical lines ever intersect the graph in more than one location.

There might be a better way to say it, but saying "a relation on a graph..." just feels a bit weird to me.