time_units: generations
demes:
- name: deme1
  epochs:
  - {start_size: 1}
- name: deme2
  epochs:
  - {start_size: 1, end_time: 50}
- name: deme3
  ancestors: [deme2]
  epochs:
  - {start_size: 1}
pulses:
- {sources: [deme1], dest: deme3, proportions: [0.9], time: 50}