Until 4874.466 generations ago, the population of deme X was 861. At that point, it changed to 6688, where it has stayed up to now.
time_units: generations
demes:
  - name: X
    start_time: .inf
    epochs:
      - start_size: 861
        end_time: 4874.466
      - start_size: 6688
        end_time: 0