Launch file with xacro parameter

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Hi all, I have some problem in simulating multi-robot systems in Gazebo/ROS. The problem is in the topic name since every robot subscribes to the same topic (e.g. for a laser in base_scan/scan). I would like to pretag it (e.g. robotname/base_scan/scan). Is there anyone with the same problem? I already tried with xacro, i.e. I create a tag of my robot (including the canonical link) but the launch file seems that does not like a macro initialization in it. Any other suggestions? Thank you for your time,

Marco

P.S. IF it could help, my launch file looks like this <launch>

<node name="gazebo" pkg="gazebo" type="gazebo" args="$(find labrob_diffDrive3d_description)/worlds/empty.world" respawn="false" output="screen"/>

<node name="spawn_kh31" 
  pkg="gazebo" type="spawn_model"
  args="-urdf -param robot_file -model robot1 -x 0 -y 0 -z 0.2"
  respawn="false" output="screen"
/>

</launch> This is the version not using the macro file