code: public static double log1mexp(double x) {
      return (x > -LOG2) ? FastMath.log(-FastMath.expm1(x)) : FastMath.log1p(-exp(x));
    }
docs: More stable than {@code FastMath.log(1 - FastMath.exp(x))}
  
  @param x Value
  @return log(1-exp(x))