code: def set_custom_boundary(doc):
      
      if doc.user_data == {}:
          raise AttributeError("A list of Sentence is not attached to doc.user_data.")
      # Set every token.is_sent_start False because they are all True by default
      for token_nr, token in enumerate(doc):
          doc[token_nr].is_sent_start = False
      # Set token.is_sent_start True when it is the first token of a Sentence
      token_nr = 0
      for sentence in doc.user_data:
          doc[token_nr].is_sent_start = True
          token_nr += len(sentence.words)
      return doc
docs: Set the sentence boundaries based on the already separated sentences.
      :param doc: doc.user_data should have a list of Sentence.
      :return doc: