extending Raphael.js: draggable sets

/*
    extending Raphael.js: draggable sets
*/

Raphael.st.draggable = function() {
  var _this = this,
      lx = 0,
      ly = 0,
      ox = 0,
      oy = 0,
      move = function(dx, dy){
          lx = dx + ox;
          ly = dy + oy;
          _this.transform('t' + lx + ',' + ly);
      },
      start = function() {  
          this.animate({opacity: 0.4}, 100, 'ease-out'); 
      },
      end = function() {
          ox = lx;
          oy = ly;
          this.animate({opacity: 1}, 200, 'ease-in');
      };
  this.attr({cursor: "move"});    
  this.drag(move, start, end);
  return this;
};