/* scroll spy plugin / class */
    var ScrollSpy = new Class({
      
      /* implements */
      Implements: [Options,Events],
    
      /* options */
      options: {
        min: 0,
        mode: 'vertical',
        max: 0,
        container: window,
        onEnter: $empty,
        onLeave: $empty,
        onTick: $empty
      },
      
      /* initialization */
      initialize: function(options) {
        /* set options */
        this.setOptions(options);
        this.container = $(this.options.container);
        this.enters = this.leaves = 0;
        this.max = this.options.max;
        
        /* fix max */
        if(this.max == 0)
        {
          var ss = this.container.getScrollSize();
          this.max = this.options.mode == 'vertical' ? ss.y : ss.x;
        }
        /* make it happen */
        this.addListener();
      },
      
      /* a method that does whatever you want */
      addListener: function() {
        /* state trackers */
        this.inside = false;
        this.container.addEvent('scroll',function() {
          /* if it has reached the level */
          var position = this.container.getScroll();
          var xy = this.options.mode == 'vertical' ? position.y : position.x;
          /* if we reach the minimum and are still below the max... */
          if(xy >= this.options.min && xy <= this.max) {
              /* trigger Enter event if necessary */
              if(!this.inside) {
                /* record as inside */
                this.inside = true;
                this.enters++;
                /* fire enter event */
                this.fireEvent('enter',[position,this.enters]);
              }
              /* trigger the "tick", always */
              this.fireEvent('tick',[position,this.inside,this.enters,this.leaves]);
          }
          else {
            /* trigger leave */
            if(this.inside)
            {
              this.inside = false;
              this.leaves++;
              this.fireEvent('leave',[position,this.leaves]);
            }
          }
        }.bind(this));
      }
    });