rospack can not find my plugin

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Hi, I am following this tutorial http://wiki.ros.org/navigation/Tutori... and write a global planner plugin. my package is robocars, my class name is robocars::PathFollowingGlobalPlanner. Here is my package.xml:

<export>
    <!-- Other tools can request additional information be placed here -->
    <!-- <nav_core plugin="${prefix}/path_following_global_planner_plugin.xml" /
>-->
    <nav_core plugin="${prefix}/path_following_global_planner_plugin.xml" />
  </export>

And here is my path_following_global_planner_plugin.xml

<library path="lib/libpath_following_global_planner_lib">
  <class name="robocars/PathFollowingGlobalPlanner" type="robocars::PathFollowin
gGlobalPlanner" base_class_type="nav_core::BaseGlobalPlanner">
    <description>This is a global planner plugin by robocars project.</descripti
on>
  </class>
 </library>

PathFollowingGlobalPlanner class compiled fine. "rospack find robocars" can find my package. But when I run "rospack plugins --attrib=plugin nav_core", the plugin did not show up.

Why