A pool finishes when its kth member finishes. Use the pool label in
add_outcome(), another pool, or after().
Examples
spec <- race_spec()
spec <- add_accumulator(spec, "A", "lognormal")
spec <- add_accumulator(spec, "B", "lognormal")
spec <- add_pool(spec, "P1", members = c("A", "B"), k = 1L)
