docs: def _format_postconditions(postconditions: List[icontract._Contract], prefix: Optional[str] = None) -> List[str]:
      """
      Format postconditions as reST.
  
      :param postconditions: postconditions of a function
      :param prefix: prefix to be prepended to ``:ensures:`` directive
      :return: list of lines describing the postconditions
      """
      if not postconditions:
          return []
  
      result = []  # type: List[str]
  
      if prefix is not None:
          result.append(":{} ensures:".format(prefix))
      else:
          result.append(":ensures:")
  
      for postcondition in postconditions:
          result.append("    * {}".format(_format_contract(contract=postcondition)))
  
      return result