time_units: generations
demes:
- name: a
  epochs:
  - {start_size: 100, end_time: 100}
- name: b
  ancestors: [a]
  start_time: 200
  epochs:
  - {start_size: 100}
migrations:
- rate: 0.1
  source: a
  dest: b
  start_time: 250