time_units: generations
defaults:
  migration: {rate: 0.1}
demes:
- name: a
  epochs:
  - {start_size: 1}
- name: b
  epochs:
  - {start_size: 1}
migrations:
- {source: a, dest: b, start_time: 300, end_time: 200}
- {rate: 0.2, source: a, dest: b, start_time: 200, end_time: 100}
- demes: [a, b]
  start_time: 30
  end_time: 20
- rate: 0.2
  demes: [a, b]
  start_time: 20
  end_time: 10