time_units: generations
demes:
  - name: X
    epochs:
      - end_time: 1000
        start_size: 2000
  - name: A
    ancestors: [X]
    epochs:
      - start_size: 2000
  - name: B
    ancestors: [X]
    epochs:
      - start_size: 2000
migrations:
  - {source: A, dest: B, rate: 1e-4, start_time: 1000, end_time: 800}
  - {source: B, dest: A, rate: 1e-3, start_time: 500, end_time: 200}
  - {source: A, dest: B, rate: 1e-5, start_time: 200, end_time: 0}