old_contents: 
  class BaseOptimizer(object):
  
      def __init__(self):
          pass
      
      def minimize(self, cost, gradient=None, variables=None):
          pass
  
  
  
new_contents: 
  class BaseOptimizer(object):
  
      def __init__(self):
          raise NotImplementedError
  
      def minimize(self, cost, gradient=None, variables=None):
          raise NotImplementedError
  
subject: Make BaseOptimizer throw NotImplementedErro