time_units: generations
defaults:
  migration: {start_time: 100}
demes:
- name: a
  epochs:
  - {start_size: 1}
- name: b
  epochs:
  - {start_size: 1}
- name: c
  epochs:
  - {start_size: 1}
- name: d
  epochs:
  - {start_size: 1}
migrations:
- {rate: 0.1, source: a, dest: b, end_time: 90}
- {rate: 0.1, source: b, dest: a, start_time: 90, end_time: 80}
- rate: 0.1
  demes: [c, d]
  end_time: 90
- rate: 0.1
  demes: [c, d]
  start_time: 90
  end_time: 80