and - this sequence is an often seen technique to expand the eye space for making life. Black is guaranteed two eyes at a and b.
(For advanced readers: usually, White should keep his options of c or d open. It is also a good ko threat.)
is quickly captured.
Connect at is insufficient - hane reduces the eye space and kills. at is met by at .
The simple stretch can't get enough eye space to live.
Referenced byBeginnerExercise52