def METHOD_NAME(expr):
    """
    Check validity of a propositional sentence.
    A valid propositional sentence is True under every assignment.
    Examples
    ========
    >>> valid(a | ~a)
    True
    >>> valid(a | b)
    False
    References
    ==========
    * https://en.wikipedia.org/wiki/Validity
    """
    return not satisfiable(Not(expr))