def zero_at(pos, size=8):
    """
    Create a size-bit int which only has one '0' bit at specific position.

    :param int pos: Position of '0' bit.
    :param int size: Length of value by bit.
    :rtype: int
    """
    assert 0 <= pos < size
    return 2**size - 2**(size - pos - 1) - 1