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I love your instruction!
I only wander if adding command so early before whole behavior is achieved is good? Dog may make wrong association of the command if something goes wrong with learning, and with chaining always something can go wrong!
Second, for a sake of instruction I would add that dog have to know how to play tug of war and I would use object which dog is familiar with to attach to the doors. Will make life a bit easier as dog will associate that rope or whatever we use with pulling and having in his mouth.
