time_units: generations
demes:
- name: ancestral
  epochs:
  - {start_size: 100}
- name: a
  ancestors: [ancestral]
  start_time: 100
  epochs:
  - {start_size: 100, end_time: 50}
- name: b
  epochs:
  - {start_size: 100}
- name: c
  ancestors: [a]
  start_time: 20
  epochs:
  - {start_size: 100}