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

    Return None if `v` is None else `not v`.

    Examples
    ========

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

    """
    if v is None:
        return v
    else:
        return not v
