How does mrpt_icp_slam use odometry?

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We want to know how a scan matching based method uses odometry for building a map. We study the mrpt_icp_slam_2d which is a icp matching based method. In its ros wiki, it claims that "Odometry is optional". To our understandings, there should be a function to fuse the odometry information stored in tf tree.

However, we read the source code here, there only a function ICPslamWrapper::updateSensorPose(std::string _frame_id) that updates transfrom between the laser sensor frame and base frame. Odom frame is not involved. There is also no ros::subscriber to deal with odom topic.

So how does mrpt_icp_slam use odometry?