const int speedup_turn = 3000;
    
    // Monster generation in the Vestibule starts ratcheting up quickly
    // after speedup_turn turns spent in the Vestibule.
    int genodds = (you.char_direction == GDT_DESCENDING) ? 240 : 8;
    if (env.turns_on_level > speedup_turn)

    // Monster generation in the Vestibule drops off quickly.
    const int taper_off_turn = 500;
    int genodds = 240;
    if (env.turns_on_level > taper_off_turn)

        genodds -= (env.turns_on_level - speedup_turn) / 14;
        if (genodds < 3)
            genodds = 3;

        genodds += (env.turns_on_level - taper_off_turn);
        genodds = (genodds < 0? 20000 : std::min(genodds, 20000));

        int distance_odds = 10;
        if (env.turns_on_level > speedup_turn)
            distance_odds -= (env.turns_on_level - speedup_turn) / 100;
        if (distance_odds < 2)
            distance_odds = 2;

            (one_chance_in(distance_odds) ? PROX_NEAR_STAIRS 
             : PROX_AWAY_FROM_PLAYER);

            (one_chance_in(10) ? PROX_NEAR_STAIRS : PROX_AWAY_FROM_PLAYER);