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

a = vector(10 000)

compute(n) = {
	my (w=0);
	for (v=1, oo,
		w += 1 + a[n-v+1];
		if (!seen(w),
			see(w);
			return (v);
		);
	);
}

for (n=1, #a, print (n " " a[n] = compute(n)))

quit