code: public static double quantile(double val, double loc, double scale, double shape) {
      return loc - scale * FastMath.log(FastMath.exp(FastMath.log(val) / -shape) - 1);
    }
docs: Quantile function.
  
  @param val Value
  @param loc Location
  @param scale Scale
  @param shape Shape
  @return Quantile