def warn_only(flag):
    """Expression constraint mutator/wrapper:

    When flag is True,  the wrapped expression was satisifed, so return True
    signaling a passed validator expression.

    If the flag is False, the expression evaluated successfully but was not
    satisified.  Return the value "W" signaling that only a warning should be
    issued rather than an exception or error.

    >>> warn_only(True)
    True

    >>> warn_only(False)
    'W'
    """
    return "W" if not flag else True