function smoothScroll(el, x, y) {
      var scrollable;
      var startX;
      var startY;
      var method;
      var startTime = now();

      // define scroll context
      if (el === d.body) {
        scrollable = w;
        startX = w.scrollX || w.pageXOffset;
        startY = w.scrollY || w.pageYOffset;
        method = original.scroll;
      } else {
        scrollable = el;
        startX = el.scrollLeft;
        startY = el.scrollTop;
        method = scrollElement;
      }

      // scroll looping over a frame
      step({
        scrollable: scrollable,
        method: method,
        startTime: startTime,
        startX: startX,
        startY: startY,
        x: x,
        y: y
      });
    }
scrolls window or element with a smooth behavior
@method smoothScroll
@param {Object|Node} el
@param {Number} x
@param {Number} y
@returns {undefined}