Re: "The starting list count" ?????

[Robin Tarsiger <rtt@dasyatidae.com>]

Alan Mackenzie wrote:
>     `#N'
>         When printing circular structures, this construct is used to
>         represent where the structure loops back onto itself, and `N' is
>         the starting list count:
> 
>              (let ((a (list 1)))
>                (setcdr a a))
>              => (1 . #0)
> 
> ..  What does this mean, please?  What does "is the starting list count"
> mean?  There is only one "list", so what is the "list count"?

I agree that this is a bit too abbreviated, but basically it's the number
you'd pass to nthcdr along with the immediately enclosing list to get what's
spliced in as that last cdr.

    (a b c . #0) ~ (a b c a b c a b c ...)
    (a b c . #1) ~ (a b c b c b c ...)
    (a b c . #2) ~ (a b c c c ...)
    (a b c . #3) ~ <can't happen>

-RTT