s = 0
unseen = 1
seen(v) = bit test(s, v)
see(v) = s = bit or(s, 2^v); while (seen(unseen), unseen++)

\\ Kolakoski's sequence
k = vector(11 000)
k[1] = 1
k[2] = 2
u = 0
for (r=1, oo, for (i=1, k[r], k[u++] = 2-(r%2); if (u==#k, break (2))))

compute(n) = {
	for (v=unseen, oo,
		if (!seen(v) && k[n]!=k[v],
			see(v);
			return (v);
		);
	);
}

for (n=1, 10 000, print (n " " compute(n)))

quit