const a = 0.9; // min=0.1, max=5, step=.01
const b = 0.1; // min=0.1, max=5, step=.1
const turtle = new Turtle();
function walk(i) {
    var theta = i*a;
    var len = i*b;
    var x = Math.sin(theta)*len;
    var y = Math.cos(theta)*len;
    turtle.goto(x, y);
    return len < 200;
}