
 x+i
x+i ((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)))
 ((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))) as,bs,cs:T*. ((as @ bs) @ cs) = (as @ (bs @ cs)))
 0
as,bs,cs:T*. ((as @ bs) @ cs) = (as @ (bs @ cs)))
 0| 1 |  (firstn(-1+x;nth_tl(i;as)) @ [nth_tl(i;as)[(-1+x)]] @ firstn(-i+k+-x;nth_tl(i+x;as)))
=
(firstn(-1+x;nth_tl(i;as)) @ firstn(1+-i+k+-x;nth_tl(-1+i+x;as))) | 
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