old_contents: 
  class ComputeRank(object):
  
      def __init__(self, var_name):
          self.var_name = var_name
  
      def __call__(self, documents):
          return documents
  
  def test_rank():
      plugin = ComputeRank("rank")
  
      docs = [{}, {}]
  
      result = plugin(docs)
  
      doc1, doc2 = result
  
      assert doc1["rank"] == 1
      assert doc2["rank"] == 2
new_contents: 
  class ComputeRank(object):
  
      def __init__(self, var_name):
          self.var_name = var_name
  
      def __call__(self, documents):
          for i, document in enumerate(documents, 1):
              document[self.var_name] = i
          return documents
  
  def test_rank():
      plugin = ComputeRank("rank")
  
      docs = [{}, {}]
  
      result = plugin(docs)
  
      doc1, doc2 = result
  
      assert doc1["rank"] == 1
      assert doc2["rank"] == 2
subject: Implement ComputeRank