Walk along a direction field

You will write the walk of the lesson as code, for the equation $\dfrac{dy}{dx} = x - y$.

The idea. At a point $(x, y)$ the equation gives the slope $x - y$. A step of width $h$ along that slope changes the height by $h \times (x - y)$. Many short steps trace a curve.

Your tasks.

1. Finish slope_at(x, y). Return the slope that the equation gives at the point.
2. Finish step(x, y, h). Return the new height after one step of width h.
3. Finish walk(x0, y0, h, n). Start at (x0, y0), take n steps, and return the final height.