code: function startTrackingPointer( pointsList, gPoint ) {
  
          // If isPrimary is not known for the pointer then set it according to our rules:
          //    true if the first pointer in the gesture, otherwise false
          if ( !gPoint.hasOwnProperty( 'isPrimary' ) ) {
              if ( pointsList.getLength() === 0 ) {
                  gPoint.isPrimary = true;
              } else {
                  gPoint.isPrimary = false;
              }
          }
          gPoint.speed = 0;
          gPoint.direction = 0;
          gPoint.contactPos = gPoint.currentPos;
          gPoint.contactTime = gPoint.currentTime;
          gPoint.lastPos = gPoint.currentPos;
          gPoint.lastTime = gPoint.currentTime;
  
          return pointsList.add( gPoint );
      }
docs: ///////////////////////////////////////////////////////////////////////////// Device-agnostic DOM event handlers ///////////////////////////////////////////////////////////////////////////// 
  @function
  @private
  @inner
  @param {OpenSeadragon.MouseTracker.GesturePointList} pointsList
  The GesturePointList to track the pointer in.
  @param {OpenSeadragon.MouseTracker.GesturePoint} gPoint
  Gesture point to track.
  @returns {Number} Number of gesture points in pointsList.