docs: def tokenize(self, docs):
          """ Tokenizes a document, using a lemmatizer.
  
          Args:
              | doc (str)                 -- the text document to process.
  
          Returns:
              | list                      -- the list of tokens.
          """
          if self.n_jobs == 1:
              return [self._tokenize(doc) for doc in docs]
          else:
              return parallel(self._tokenize, docs, self.n_jobs)