[FFmpeg-user] Cannot find a matching stream for unlabeled input pad 7 on filter Parsed_concat_2

[Randy Johnson]

On October 29, 2020, Nicolas George <george at nsup.org> wrote:
> Randy Johnson via ffmpeg-user (12020-10-29):
> > "[0:v]setsar=1[v0];[2:v]setsar=1[v1];
>
> > [v0]
>
> 1
>
> > [4:a]
>
> 2
>
> > [v1]
>
> 3
>
> > [4:a]
>
> 4
>
> > concat=n=4:v=1:a=1"
>
> 4×(1+1) = 8 ≠4
>

Can you please elaborate on the (1+1) part?
I see here https://trac.ffmpeg.org/wiki/Concatenate 

that n=x where x is the number of input segments
>

Thank You,
Randy