For deme U, the population count was 8381 until a change 8581.844 generations ago, where it changed to 7026, remaining steady thereafter.
time_units: generations
demes:
  - name: U
    start_time: .inf
    epochs:
      - start_size: 8381
        end_time: 8581.844
      - start_size: 7026
        end_time: 0