Cut the chicken breats into thin strips or cubes. Bring the stock to a boil. Add the soy sauces, mirin, sake or dry sherry, and sugar, Bring back to a boil. Add the chicken, reduce the heat and simmer for 5 minutes. Stir in the scallions and leave stand for 1 minute. Lightly stir the eggs in a bowl just enough to break them up, Add the eggs tot the pan and leave to stand for a further 2 minutes, Carefull stir once. Spoon hot rice into indiidual bowls. Serve immediately