anchor.diff:      currentSpeed *= 2;
     }
   }
  +----
   
  
positive.code: 
  The following example indicates that anything that satisfies the `Robot` interface can be moved, can give its current position (as an integer), and can accelerate. But it does not say how any of this is done, internally.
  
  ---
  public interface Robot {
    public void move(int steps);
    
  
negative.code:   
    public void accelerate();
  }
  ----
  
  Here is an example of a class implementing this interface.
  
  ----
  public class FastRobot {
    private int currentSpeed;
    
  