Exception for subtraction fraction

Math

\[\frac{-\left(f + n\right)}{f - n}\]

C

double f(double f, double n) {
        double r7970 = f;
        double r7971 = n;
        double r7972 = r7970 + r7971;
        double r7973 = -r7972;
        double r7974 = r7970 - r7971;
        double r7975 = r7973 / r7974;
        return r7975;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019191 
(FPCore (f n)
  :name "subtraction fraction"
  (/ (- (+ f n)) (- f n)))

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