Exception for fabs fraction 2

Math

\[\frac{\left|a - b\right|}{2}\]

C

double f(double a, double b) {
        double r8069 = a;
        double r8070 = b;
        double r8071 = r8069 - r8070;
        double r8072 = fabs(r8071);
        double r8073 = 2.0;
        double r8074 = r8072 / r8073;
        return r8074;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019191 
(FPCore (a b)
  :name "fabs fraction 2"
  (/ (fabs (- a b)) 2.0))

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