Action
Draws the level curves: f(x,y) = c for several values of level c chosen equally spaced in the range interval.
levels x^2 + y^2 30 3 [10,20]
Draws 3 concentric circles with center at (0,0), beginning with the circle x² + y² = 10, then x² + y² = 15 ending with the circle x² + y²= 20