Chapter 2, Pythagorean theorem

The Pythagorean theorem is `a^2 + b^2 = c^2`. `w` and `h` are bad choices and confused me as they appear to represent height and width, not the two sides of a right triangle.

So I suggest that you change the exercise to expect this, to be clearer:

```ps
diagonal a b = sqrt (a * a + b * b)
```