code: def _update_pdf(population, fitnesses, pdfs, quantile):
      
      # First we determine a fitness threshold based on a quantile of fitnesses
      fitness_threshold = _get_quantile_cutoff(fitnesses, quantile)
  
      # Then check all of our possible pdfs with a stochastic program
      return _best_pdf(pdfs, population, fitnesses, fitness_threshold)
docs: Find a better pdf, based on fitnesses.