Instead of evaluating one variable, x, on the range (0, )
We evaluate two, x and y
So we're asking what values z assumes on the entire quarter-plane
Fortunately, it's enough to know its values only at the four corners [0, 0], [ , 0], [0, ], [ , ]
These values are merely a/e, b/f, c/g, and d/h
If all four agree on the next term, z can output it
Otherwise, input from x or y
Choose from x if |b/f - a/e| > |c/g - a/e|