code: function begin_gesture(e) {
          var touch = e.touches[0];
          gesture_in_progress = true;
          gesture_start_x = touch.clientX;
          gesture_start_y = touch.clientY;
      }
docs: Begins a new gesture at the location of the first touch in the given
  touch event.
  
  @private
  @param {TouchEvent} e The touch event beginning this new gesture.