>>> def count_binary():
    ...     yield "1"
    ...     for prefix in count_binary():
    ...         yield prefix + "0"
    ...         yield prefix + "1"
    ...
    >>> cb = count_binary()
    >>> next(cb)
    '1'
    >>> next(cb)
    '10'
    >>> next(cb)
    '11'
    >>> next(cb)
    '101'