time_units: generations
demes:
  - name: X
    epochs:
      - start_size: 2000
  - name: Y
    epochs:
      - start_size: 2000
        end_time: 1000
  - name: A
    ancestors: [X, Y]
    proportions: [0.1, 0.9]
    start_time: 1000
    epochs:
      - start_size: 2000