Bring soup stock to a boil, add shredded pork, black mushrooms and wood ears. Cook 2-3 minutes. Add remainder of ingredients and seasonings (except cornstarch, eggs, and green onion) reduce heat and simmer for 2 minutes. Thicken with cornstarch and turn off heat. Slowly pour in beaten eggs in a thin stream while stirring. Serve immediately. Garnish with green onion. If soup is to be prepared ahead of time, do not add cornstarch and eggs until serving time. Otherwise the egg will be overcooked and spoil the appearance. Soup should be quite hot and sour. Adjust the hotness with varying amount of white pepper and the sourness with different amounts of vinegar.