code: function(t) {
      return {
        id: t.identifier,
        y: t.clientY,
        x: t.clientX,
      };
    }
docs: Extract the fields from the Touch event that we need.  If we saved the event directly, it has references to other objects (like x-row) that might stick around for a long time.  This way we only have small objects in our lastTouch_ state.