code: from llama_index.core import SimpleDirectoryReader
  from llama_index.core.node_parser import SentenceSplitter
  from llama_index.core.extractors import SummaryExtractor
  
  reader = SimpleDirectoryReader('files')
  documents = reader.load_data()
  parser = SentenceSplitter(include_prev_next_rel=True)
  nodes = parser.get_nodes_from_documents(documents)
  
  summary_extractor = SummaryExtractor(
      summaries=["prev", "self", "next"]
  ) 
  metadata_list = summary_extractor.extract(nodes)
  
  print(metadata_list)
apis: llama_index.core.node_parser.SentenceSplitter llama_index.core.extractors.SummaryExtractor llama_index.core.SimpleDirectoryReader
extract_api: [((176, 206), 'llama_index.core.SimpleDirectoryReader', 'SimpleDirectoryReader', (['"""files"""'], {}), "('files')\n", (197, 206), False, 'from llama_index.core import SimpleDirectoryReader\n'), ((247, 291), 'llama_index.core.node_parser.SentenceSplitter', 'SentenceSplitter', ([], {'include_prev_next_rel': '(True)'}), '(include_prev_next_rel=True)\n', (263, 291), False, 'from llama_index.core.node_parser import SentenceSplitter\n'), ((364, 416), 'llama_index.core.extractors.SummaryExtractor', 'SummaryExtractor', ([], {'summaries': "['prev', 'self', 'next']"}), "(summaries=['prev', 'self', 'next'])\n", (380, 416), False, 'from llama_index.core.extractors import SummaryExtractor\n')]