def fuzzy_not(arg):
    """
    Not in fuzzy logic

    Will return Not if arg is a boolean value, and None if argument
    is None.

    Examples:

    >>> from sympy.logic.boolalg import fuzzy_not
    >>> fuzzy_not(True)
    False
    >>> fuzzy_not(None)
    >>> fuzzy_not(False)
    True

    """
    if arg is None:
        return
    return not arg