@type t() :: %Imp.Training.FastSlow.PromptPopulation{
  anchor_digest: String.t() | nil,
  candidate_ids: [String.t()],
  candidates: [Imp.Training.FastSlow.Config.json_value()],
  digest: String.t(),
  instance_frontier: %{required(String.t()) => [String.t()]},
  instance_scores: %{
    required(String.t()) => %{required(String.t()) => number()}
  },
  lookahead_digest: String.t() | nil,
  parent_digest: String.t() | nil,
  revision: non_neg_integer()
}