python - Understanding simulation termination -

this part of simulation describes program termination. please me understand how part of simulation work lost. code below:

while time_elapsed < end_time :     event=birth () +death () +infection()     choice=random.random()*event     choice -= birth()     time_elapsed += random.expovariate(event)      if choice < 0 :         do_birth()         continue     choice -= death()     if choice < 0:         do_death()         continue 

i've rearranged make more obvious happening:

# @ each time-step until sim ends while time_elapsed < end_time :      #     # decide random event occur     #      # cumulative probability = (chance of birth) + (chance of death) + (chance of infection)     event = birth() + death() + infection()     # choose value in [0..cum_prob)     choice = random.random() * event      # interpret value:     # if 0 <= value < (chance of birth)  =>  birth     # if (chance of birth) <= value < (chance of birth) + (chance of death)   => death     # if (chance of birth) + (chance of death) <= value < (chance of birth) + (chance of death) + (chance of infection)   => infection      choice -= birth()     if choice < 0 :         do_birth()         continue      choice -= death()     if choice < 0:         do_death()         continue      # "what in case of infection" missing;     # assume chopped off while cut-and-pasting.      # forward next time-step;     #   uses exponential step-size, giving     #   normal distribution of event intervals     #   rather uniform step size     time_elapsed += random.expovariate(event) 

i write bit differently:

while < stop_at:     birth_point =               prob_birth()     death_point = birth_point + prob_death()     all_events  = death_point + prob_infection()      event = random.random() * all_events      if event < birth_point:         do_birth()     elif event < death_point:         do_death()     else:         do_infection()      += random.expovariate(all_events)