How to manipulate joints in a simple legged model

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What's the easiest way to programmatically manipulate a legged model? Through the excellent "build a robot" tutorial, I was able to build a simple four-legged platform. How would I manipulating the limbs to test different gaits?

I've read through some of the other tutorials, and I'm a little confused on how to proceed. Do I write a model plugin, or connect it to ROS...or both?

I'm not trying to do anything fancy. I just want to be able to write a simple program to extend/retract/rotate legs in different sequences.