Ivan Detkov:
It is elementary. Let's keep it.

caranthir:
After black has 3 straight, the shape is "bent four in the corner", which is a dead shape. In Japanese rules this is stipulated ad hoc, whereas in all Chinese-style rulesets, compatible with area scoring, it directly follows from the rules that the shape is dead. In the latter, the attacker can, at the end of the game after filling all dame and having removed all outside liberties of white's group, then remove all remaining ko threats - if any - from their position (without loss of points; that's why area scoring matters), then play at A18 (top left coordinates) to enforce a ko which they cannot lose. That is: b A18, w D19 takes, b B19, w A19, b A18 takes and creates the ko.