time_units: generations
defaults:
  epoch: {start_size: 1}
demes:
- name: a
  epochs:
  - {end_time: 200}
- name: b
  ancestors: [a]
  start_time: 200
  epochs:
  - {end_time: 100}
- name: c
  ancestors: [b]
  start_time: 100