Lonestar: I think the solution must be the play at .
fivebells?, beginner: This may be obvious to others, but I came over here to ask whether the hane at works, and determined why it doesn't while writing my reasoning out: After black a, b and c are miai for an eye, and black is already guaranteed one eye by a play at d or e.
JoelR: No, if Black responds at a, White plays f, transposing to the correct line. But if Black plays f, then he protects his four stones, and White cannot stop the eye along the edge.
Gabriel: Beginner here. I advocate playing to falsify the eye at a, forcing black to make both eyes at the bottom.
...unless black plays at b.
Superdave: Black would not play at b because then White would get another chance to play out the correct solution. White at b is not sente, so Black actually needs "one-and-a-half" eyes at the bottom; that is, one for sure, and another one depending on who moves first. I think Black d works; either White hanes at e and Black plays b, or the other way around; either way Black has two eyes and lives.