Exception for Quotient of sum of exps

Math

\[\frac{e^{a}}{e^{a} + e^{b}}\]

C

double f(double a, double b) {
        double r8218 = a;
        double r8219 = exp(r8218);
        double r8220 = b;
        double r8221 = exp(r8220);
        double r8222 = r8219 + r8221;
        double r8223 = r8219 / r8222;
        return r8223;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019191 
(FPCore (a b)
  :name "Quotient of sum of exps"

  :herbie-target
  (/ 1.0 (+ 1.0 (exp (- b a))))

  (/ (exp a) (+ (exp a) (exp b))))

Backtrace

get-representation: Unknown representation #f		L	C
loop	/data/pavpan/nightlies/herbie/interface2/src/points.rkt	122	4
prepare-points	/data/pavpan/nightlies/herbie/interface2/src/points.rkt	146	0
setup-prog!34	/data/pavpan/nightlies/herbie/interface2/src/mainloop.rkt	67	0
run-improve43	/data/pavpan/nightlies/herbie/interface2/src/mainloop.rkt	339	0
(unnamed)	/opt/racket-7.0/collects/racket/private/more-scheme.rkt	261	28
run	/opt/racket-7.0/share/pkgs/profile-lib/main.rkt	39	2
profile-thunk16	/opt/racket-7.0/share/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/opt/racket-7.0/collects/racket/private/more-scheme.rkt	261	28