time_units: generations
demes:
- name: a
  epochs:
  - {end_time: 100, start_size: 10}
  - {start_size: 20, end_time: 0}
- name: x
  ancestors: [a]
  start_time: 100
  epochs:
  - {start_size: 100}
- name: "y"
  epochs:
  - {start_size: 100}
- name: z
  ancestors: [a]
  start_time: 100
  epochs:
  - {start_size: 100, end_time: 10}
  - {start_size: 10}