time_units: generations
demes:
- name: a
  defaults:
    epoch: {start_size: 1}
- name: b
  epochs:
  - {end_time: 90}
  - {end_size: 100, end_time: 50}
  - {start_size: 100, end_size: 50}
  defaults:
    epoch: {start_size: 1}