docs: def keys(source, ext=COMPLETE):
      """Extracts metadata keys from the provided MultiMarkdown text.
  
      Keyword arguments:
      source -- string containing MultiMarkdown text
      ext -- extension bitfield for extracting MultiMarkdown
      """
      _MMD_LIB.extract_metadata_keys.restype = ctypes.c_char_p
      _MMD_LIB.extract_metadata_keys.argtypes = [ctypes.c_char_p, ctypes.c_ulong]
      src = source.encode('utf-8')
      all_keys = _MMD_LIB.extract_metadata_keys(src, ext)
      all_keys = all_keys.decode('utf-8') if all_keys else ''
      key_list = [ii for ii in all_keys.split('\n') if ii]
      return key_list