time_units: generations
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: 0.01
  demes: [a, b]
- rate: 0.01
  demes: [b, c]
- rate: 0.01
  demes: [a, c, d]
- {rate: 0.01, source: b, dest: d}