For deme U, the population count was 129 until a change 2213.216 generations ago, where it grew to 3476, remaining steady thereafter.
time_units: generations
demes:
  - name: U
    start_time: .inf
    epochs:
      - start_size: 129
        end_time: 2213.216
      - start_size: 3476
        end_time: 0