input: Write a function to parse a JSON data structure and return the values of the 'name', 'age', and 'occupation' fields. If any of these fields are missing, the function should return an error message.
  def parse_json(data):
      try:
          data = json.loads(data)
          name = data['name']
          age = data['age']
          occupation = data['occupation']
          return name, age, occupation
      except KeyError:
          return "Error: One or more required fields are missing."
  
  assert parse_json('{"name": "John", "age": 18, "occupation": "Software Engineer"}')==('John', 18, 'Software Engineer')
  assert parse_json('{"name": "Alice", "occupation": "Doctor"}')=="Error: One or more required fields are missing."
output: def parse_json(data):
      try:
          data = json.loads(data)
          name = data['name']
          age = data['age']
          occupation = data['occupation']
          return name, age, occupation
      except KeyError:
          return "Error: One or more required fields are missing."