Gray Area—good question, thanks for bring my attention to reservoir sampling. I found a compact description of it in Devroye’s “Non-Uniform …” and for sampling just 1 integer x it looks as follows
At step 1, let x=1
At step k, let x=k with probability 1/k
sum_i 1/i diverges, this means x will never stop growing
pdf23ds—I think you can use reservoir sampling for sampling from infinite streams, an interesting question is when it works. For instance, consider an infinite stream of IID bits, 1-element reservoir sampling converges after 1 step. An interesting question is when exactly it works—my intuition is that it works whenever the stream has finite entropy, and a stationary Markov property
Disallowing a symbol for “all events” breaks the definition of a probability space. It’s probably easier to allow extended reals and break some field axioms than figure out do rigorous probability without a sigma-algebra.