Dieter: many variations to be added.
The cannot live on their own.
Hence, it is fortunate that black stones by the sequence above place the white stones in atari
from which they () cannot be saved by connecting with 'a' to the next group ,
because it () has only two liberties 'a' and 'b'. The connection would fill one of the liberties and Black () could take the whole group with 'b'.