time_units: generations
demes:
- name: a
  epochs:
  - {start_size: 100}
- name: b
  epochs:
  - {start_size: 200}
- name: c
  epochs:
  - {start_size: 300}
- name: d
  epochs:
  - {start_size: 400, end_time: 0}
migrations:
- rate: 0.01
  demes: [a, b, c]
- rate: 0.01
  demes: [a, d]