big = 1 000 000
s = 0
S = Set([])
unseen = 1
seen(v) = if (v < big, bit test(s, v), set search(S, v))
see(v) = if (v < big, s = bit or(s, 2^v), S = set union(S, Set([v]))); while (seen(unseen), unseen++)

other(n, p) = {
	see(p);
	my (w=#binary(p), v);

	\\ invert leftmost binary digit ?
	forstep (b=w-1, 0, -1,
		v = bitxor(p, 2^b);
		if (!seen(v),
			print (n " " b);
			return (v);
		);
	);

	\\ invert rightmost leading zero ?
	for (b=w, oo,
		v = bitxor(p, 2^b);
		if (!seen(v),
			print (n " " b);
			return (v);
		);
	);
}

v=0; for (n=0, 2^13-1, v=other(n, v))

quit