Exception for Cubic critical

Math

\[\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a}\]

C

double f(double a, double b, double c) {
        double r81536 = b;
        double r81537 = -r81536;
        double r81538 = r81536 * r81536;
        double r81539 = 3.0;
        double r81540 = a;
        double r81541 = r81539 * r81540;
        double r81542 = c;
        double r81543 = r81541 * r81542;
        double r81544 = r81538 - r81543;
        double r81545 = sqrt(r81544);
        double r81546 = r81537 + r81545;
        double r81547 = r81546 / r81541;
        return r81547;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019323 
(FPCore (a b c)
  :name "Cubic critical"
  :precision binary64
  (/ (+ (- b) (sqrt (- (* b b) (* (* 3 a) c)))) (* 3 a)))

Backtrace

get-representation: Unknown representation real		L	C
(unnamed)	/data/pavpan/nightlies/herbie/fix-interface-bugs/src/core/regimes.rkt	65	3
filter	/opt/racket-7.2/collects/racket/private/list.rkt	256	2
infer-splitpoints	/data/pavpan/nightlies/herbie/fix-interface-bugs/src/core/regimes.rkt	34	0
get-final-combination	/data/pavpan/nightlies/herbie/fix-interface-bugs/src/mainloop.rkt	369	0
(unnamed)	/opt/racket-7.2/collects/racket/private/more-scheme.rkt	261	28
run	/opt/racket-7.2/share/pkgs/profile-lib/main.rkt	39	2
profile-thunk16	/opt/racket-7.2/share/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/opt/racket-7.2/collects/racket/private/more-scheme.rkt	261	28