time_units: generations
demes:
- name: a
  epochs:
  - {start_size: 1}
- name: b
  epochs:
  - {start_size: 1}
pulses:
- {sources: [a], dest: b, proportions: [0.1], time: 100}
- {sources: [a], dest: b, proportions: [0.2], time: 200}