That’s a shame, because uncertainty about the number of possible outcomes is a real and challenging statistical problem. See for example Inference for the binomial N parameter: A hierarchical Bayes approach(abstract)(full paper pdf) by Adrian Raftery. Raftery’s prior for the number of outcomes is 1/N, but you can’t use that for coherent betting.
That’s a shame, because uncertainty about the number of possible outcomes is a real and challenging statistical problem. See for example Inference for the binomial N parameter: A hierarchical Bayes approach (abstract)(full paper pdf) by Adrian Raftery. Raftery’s prior for the number of outcomes is 1/N, but you can’t use that for coherent betting.