\[2 \cdot \tan^{-1} \left(\sqrt{\frac{1 - x}{1 + x}}\right)\]

2 \cdot \tan^{-1} \left(\sqrt{\frac{1 - x}{1 + x}}\right)

double f(double x) {
        double r27107 = 2.0;
        double r27108 = 1.0;
        double r27109 = x;
        double r27110 = r27108 - r27109;
        double r27111 = r27108 + r27109;
        double r27112 = r27110 / r27111;
        double r27113 = sqrt(r27112);
        double r27114 = atan(r27113);
        double r27115 = r27107 * r27114;
        return r27115;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019323 
(FPCore (x)
  :name "arccos"
  :precision binary64
  (* 2 (atan (sqrt (/ (- 1 x) (+ 1 x))))))

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