docs: def value(source, key, ext=COMPLETE):
      """Extracts value for the specified metadata key from the given extension set.
  
      Keyword arguments:
      source -- string containing MultiMarkdown text
      ext -- extension bitfield for processing text
      key -- key to extract
      """
      _MMD_LIB.extract_metadata_value.restype = ctypes.c_char_p
      _MMD_LIB.extract_metadata_value.argtypes = [ctypes.c_char_p, ctypes.c_ulong, ctypes.c_char_p]
  
      src = source.encode('utf-8')
      dkey = key.encode('utf-8')
  
      value = _MMD_LIB.extract_metadata_value(src, ext, dkey)
      return value.decode('utf-8') if value else ''