A helper function which waits for user multi-line input.

    :return: A string input by user, separated by ``'\\n'``.

def _user_input() -> str:
    """
    A helper function which waits for user multi-line input.

    :return: A string input by user, separated by ``'\\n'``.
    """

    lines = []
    try:
        while True:
            line = input()
            if line != '':
                lines.append(line)
            else:
                break
    except (EOFError, KeyboardInterrupt):
        return '\n'.join(lines)