How to load the world save by ros gazebo on groovy?

edit

I followed Tutorials/1.9/Using roslaunch Files to Spawn Models

My test.launch

<launch>

    <!-- start gazebo with an empty plane -->
    <param name="/use_sim_time" value="true" />
    <include file="$(find pr2_gazebo)/launch/pr2_empty_world.launch">
        <arg name="world_name" value="/home/sam/code/groovy_overlay/src/gazebo_dir/a.world"/>
    </include>

    <!-- start gui -->
    <node name="gazebo_gui" pkg="gazebo" type="gui" respawn="false" output="screen"/>

</launch>

a.world is created by ROS groovy gazebo(use save as).

Then I run optirun roslaunch ./test.launch , it shows:

sam@sam:~/code/groovy_overlay/src/gazebo_dir$ optirun roslaunch ./test.launch 
... logging to /home/sam/.ros/log/de7b2aec-3998-11e3-a682-aeef2ab08fe6/roslaunch-sam-21514.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

unused args [world_name] for include of [/opt/ros/groovy/stacks/pr2_simulator/pr2_gazebo/launch/pr2_empty_world.launch]
sam@sam:~/code/groovy_overlay/src/gazebo_dir$

I notice that doc says: the worldname is with respect to GAZEBORESOURCE_PATH environmental variable

But I can't understand what it means.

How to solve it?

Thank you~