TherapieGo: I agree with caranthir, the rule says "The same situation cannot occur twice in a game", so when White takes on E19, Black should not be allowed to take on D19 as we have already seen this situation. The right and only answer to this problem should therefore be to start with C18 and launch the ko.
caranthir: I really depends on the ruleset. The most relevant rulesets that either include superko or otherwise forbid the eternal life cycle are AGA, NZ and Ing; others don't. If the ruleset says that the game be void when eternal life or other similar long cycle occurs, then the defender has that option if they otherwise think ko isn't good enough to win the game.
caranthir: At least with the sequence given in the solution, w (defender) would not live under any superko rule - neither positional nor situational superko - ie. AGA, NZ, or even (strictly interpreted) Chinese rules. White would not be allowed to recapture with D19 right after b E19 (that would recreate earlier board position, also with the same player to move), and would have to play some non-local threat first (I can't find any local ones). Anyway, after using a threat, b could just play F19 (and likewise over and over again), so w could not even live in ko.
d4rkm4tter: I didn't know that. Lets have this here as some interesting information.
I somehow broke this while editing. Can't open this again. Can't even write a new message. Sorry! Help!