old_contents: def add(a, b):
      pass
  
  def sub(a, b):
      pass
  
  def multi(a, b):
      pass
  
  def div(a, b):
      pass
  
  def mod(a, b):
      pass
  
  if __name__ == '__main__':
      pass
  
new_contents: def add(a, b):
      return a+b
  
  def sub(a, b):
      pass
  
  def multi(a, b):
      pass
  
  def div(a, b):
      pass
  
  def mod(a, b):
      pass
  
  if __name__ == '__main__':
      pass
  
subject: modify add