time_units: generations
defaults:
  migration: {source: a}
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, dest: b}
- {rate: 0.1, dest: c}
- {rate: 0.1, dest: d}
- {rate: 0.2, source: d, dest: a}