This is usually expressed in the following way, which is not always easy to understand. In other words, if it appears that two different x values share the same y value, then it must turn out that they are actually one and the same x value. Don't worry if you're a little baffled. Lets explore this with Maple. That means the values of a and b are the same.
Lets look at the graph.
Just like we would do by hand, we interchange x and y, and solve for y. We then turn this solution into a function using the unapply command. Lets see how this works in action. This is called 'restricting the domain'. Lets take a peek at the sine and arcsin, and see how they are reflections of one another.