If we only use domain-specific data to train the NMT model for such a domain, the performance of resulting model is usually unsatisfying.
more specifically, at the k-th iteration, we first transfer the translation knowledge of the previous in-domain NmT model θ out trained on D out (Line 6), and then reversely transfer the translation knowledge encoded by θ Obviously, during the above procedure, one of important steps is how to transfer the translation knowledge from one domain-specific NmT model to the other one.