
 x+i
x+i (firstn(x;nth_tl(i;as)) @ firstn(k-(x+i);nth_tl(x+i;as)))
=
(firstn(x-1;nth_tl(i;as)) @ firstn(k-(x-1+i);nth_tl(x-1+i;as)))
 (firstn(x;nth_tl(i;as)) @ firstn(k-(x+i);nth_tl(x+i;as)))
=
(firstn(x-1;nth_tl(i;as)) @ firstn(k-(x-1+i);nth_tl(x-1+i;as)))| 1 |  ((firstn(-1+x;nth_tl(i;as)) @ [nth_tl(i;as)[(-1+x)]]) @ firstn(k-(x+i);nth_tl(x+i;as)))
=
(firstn(x-1;nth_tl(i;as)) @ firstn(k-(x-1+i);nth_tl(x-1+i;as))) | 
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