Joseki-related life-and-death example 6
Found at Beginner Move Function Problem 3 / Solution
The exchange of and is usually considered a bad way for Black to close the corner, because it leaves aji at the 3-3 point. I guess, that in most cases, it is better to play at a or b instead of if he wants to secure territory.(Playing might be useful to make White heavy sometimes.)
White uses the aji with an 3-3 invasion.
Sequences with blocking at here result in a local loss, leaving black overconcentrated.
If Black has helping stones nearby, he might try to avoid the ko, with heavy fighting after .
Varations given by SnotNose and Alex Weldon
Anotation by Klaus