hi I was just curious whether it is possible to take user input in sdf file that we use to create models in Gazbeo and use them to calculate something like robot inertia. I read about embedded ruby can make it possible from here. but I am still not sure how can I use embedded ruby for this. At least I need to know how I can declare a variable and initialize it and use it in arithmetic operation. please tell me the steps from writing the erb template code to how to use it in the gazebo as a world file. thankyou