The population of deme Y held at 598 until 3128.9 generations back, when it changed reaching 6811, which has been constant since then.
time_units: generations
demes:
  - name: Y
    start_time: .inf
    epochs:
      - start_size: 598
        end_time: 3128.9
      - start_size: 6811
        end_time: 0