def _is_true(x):
    """
    _is_true : thing -> boolean
    _is_true : number -> boolean
    Determines whether the argument is true.
    Returns None when attempting to assert a non-boolean

    Examples:
    >>> _is_true(True)
     False

    >>> _is_true("hi")
     None

    >>> _is_true(False)
     False
    """

    if not isinstance(x, bool):
        return None
    else:
        return x