time_units: generations
demes:
- name: a
  epochs:
  - {start_size: 1}
  defaults:
    epoch: {end_time: 10}
- name: b
  epochs:
  - {start_size: 1, end_time: 90}
  - {start_size: 2, end_time: 50}
  - {start_size: 3}
  defaults:
    epoch: {end_time: 10}