TurtleWorld.equals&diffTo: synchronized on shapes
the compared shape may not be a Line
TurtleWorld: order the points in each segment so that the line (ab, cd) is detected as equal to (cd, ab)
recursion.Circle: kill exercise. It is superseeded by turtleart
add some size hints to that star
recursion.Star: kill exercise. It is superseeded by turtleart.