def fold_description(fold: float) -> str:
    """
    Return a string for describing a fraction (including a percent representation).
    """
    if fold is None:
        return "None"

    if fold >= 0:
        value = 2 ** fold
        char = "X"
    else:
        value = 2 ** -fold
        char = "/"

    return f"{fold:.4g} ({char} {value:.4g})"