time_units: generations
demes:
- name: a
  epochs:
  - {start_size: 100, end_time: 50}
- name: b
  epochs:
  - {start_size: 100, end_time: 50}
- name: c
  ancestors: [a, b]
  start_time: 100
  epochs:
  - {start_size: 100}