code: public static double cdf(double val, double loc, double scale, double shape) {
      val = (val - loc) / scale;
      return FastMath.pow(1. + FastMath.exp(-val), -shape);
    }
docs: Cumulative density function.
  
  @param val Value
  @param loc Location
  @param scale Scale
  @param shape Shape
  @return CDF