anchor.diff:        }
       }
   
  -    if (api.goto(target)) {
  +    if (api.goto(target) != null) {
         event.stopImmediatePropagation();
         event.preventDefault();
       }
  
positive.code:             (target !== document.documentElement))) {
        target = target.parentNode;
      }
      if (api.goto(target)) {
        event.preventDefault();
      }
    });
  
negative.code:   };
  }
  
  class impress {
    Element goto(target, [duration=0]) {
      return null;
    }
    
    Element prev() {
      return null;
    }
    
    Element next() {
      return null;
    }
  }
  
  void main() {
    impress api = new impress();
    
    // prevent default keydown action when one of supported key is pressed
    document.on.keyDown.add((event) {
      if (event.keyCode === 9 || (event.keyCode >= 32 && event.keyCode <= 34 ) || (event.keyCode >= 37 && event.keyCode <= 40)) {
  