\[\lambda_1 + \tan^{-1}_* \frac{\cos \phi_2 \cdot \sin \left(\lambda_1 - \lambda_2\right)}{\cos \phi_1 + \cos \phi_2 \cdot \cos \left(\lambda_1 - \lambda_2\right)}\]

\lambda_1 + \tan^{-1}_* \frac{\cos \phi_2 \cdot \sin \left(\lambda_1 - \lambda_2\right)}{\cos \phi_1 + \cos \phi_2 \cdot \cos \left(\lambda_1 - \lambda_2\right)}

double f(double lambda1, double lambda2, double phi1, double phi2) {
        double r8267 = lambda1;
        double r8268 = phi2;
        double r8269 = cos(r8268);
        double r8270 = lambda2;
        double r8271 = r8267 - r8270;
        double r8272 = sin(r8271);
        double r8273 = r8269 * r8272;
        double r8274 = phi1;
        double r8275 = cos(r8274);
        double r8276 = cos(r8271);
        double r8277 = r8269 * r8276;
        double r8278 = r8275 + r8277;
        double r8279 = atan2(r8273, r8278);
        double r8280 = r8267 + r8279;
        return r8280;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019179 +o rules:numerics
(FPCore (lambda1 lambda2 phi1 phi2)
  :name "Midpoint on a great circle"
  (+ lambda1 (atan2 (* (cos phi2) (sin (- lambda1 lambda2))) (+ (cos phi1) (* (cos phi2) (cos (- lambda1 lambda2)))))))

Backtrace

result arity mismatch; expected number of values not received expected: 2 received: 1 in: local-binding form values...: 0		L	C
loop	/data/pavpan/nightlies/herbie/various-cleanup/src/core/extraction.rkt	25	2
(unnamed)	/data/pavpan/nightlies/herbie/various-cleanup/src/core/simplify.rkt	26	0
simplify!	/data/pavpan/nightlies/herbie/various-cleanup/src/mainloop.rkt	217	0
run-improve43	/data/pavpan/nightlies/herbie/various-cleanup/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