time_units: generations
demes:
- name: a
  epochs:
  - {start_size: 1}
  defaults:
    epoch: {size_function: constant}
- name: b
  epochs:
  - {start_size: 1, end_time: 90, size_function: constant}
  - {start_size: 100, end_size: 100, end_time: 50}
  - {start_size: 50, end_size: 50, end_time: 10}
  - {start_size: 100, end_size: 50, size_function: exponential}
  defaults:
    epoch: {size_function: exponential}