The solution is the nose tesuji of .
must go down. White cannot escape upward, as after the escape sequence a, b, c, d, e, the potential ladder in the first failure diagram (after in that diagram) would work.
With -, Black pushes white around in a loose ladder.
is an important tesuji: if played immediately at , Black plays at himself, and White has only two liberties. Now , etc. repeat the same maneuver on the right, and we get to...
Now for the really tricky part (it is easier if White captures): the incredible , breaking the symmetry at last!! And Black concludes with a double atari at b, on the last point of symmetry. :-)