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:
  - demes: [A, B]
    rate: 1e-4