20. Tightrope

• There are two unmoving blue circles on the screen.

• You can move a square with the mouse.

• Square color is:

  • Green if the distance from the square to circle 1 and the distance from the square to circle 2 are within 20 of each other.
  • Red otherwise.

Some people had trouble understanding this description. In a computer-like language:

a = distance from square to circle 1
b = distance from square to circle 2
green if a and b are within 20 of each other

Write:

1. Model
2. The function square-color: model -> color.