docs: private static double PhiInverse(double y, double delta, double lo, double hi) {
          double mid = lo + (hi - lo) / 2;
          if (hi - lo < delta) return mid;
          if (Phi(mid) > y) return PhiInverse(y, delta, lo, mid);
          else              return PhiInverse(y, delta, mid, hi);
      }