?

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

(FPCore (f n) :precision binary64 (/ (- (+ f n)) (- f n)))

double code(double f, double n) {
	return -(f + n) / (f - n);
}

real(8) function code(f, n)
    real(8), intent (in) :: f
    real(8), intent (in) :: n
    code = -(f + n) / (f - n)
end function

public static double code(double f, double n) {
	return -(f + n) / (f - n);
}

def code(f, n):
	return -(f + n) / (f - n)

function code(f, n)
	return Float64(Float64(-Float64(f + n)) / Float64(f - n))
end

function tmp = code(f, n)
	tmp = -(f + n) / (f - n);
end

code[f_, n_] := N[((-N[(f + n), $MachinePrecision]) / N[(f - n), $MachinePrecision]), $MachinePrecision]

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

Reproduce?

Please include this information when filing a bug report:

herbie shell --seed 2023243 
(FPCore (f n)
  :name "subtraction fraction"
  :precision binary64
  (/ (- (+ f n)) (- f n)))

_arb-ptr: contract violation expected: _arb? given: '(ival-illegal)		L	C
_arb-ptr	unknown
arb->ival	/home/nightlies/herbie/arb-support/src/arb.rkt	160	0
search-step	/home/nightlies/herbie/arb-support/src/searchreals.rkt	37	0
find-intervals	/home/nightlies/herbie/arb-support/src/searchreals.rkt	87	0
make-sampler	/home/nightlies/herbie/arb-support/src/sampling.rkt	94	0
sample-points	/home/nightlies/herbie/arb-support/src/ground-truth.rkt	64	0
setup-context!	/home/nightlies/herbie/arb-support/src/mainloop.rkt	293	0
get-alternatives/report	/home/nightlies/herbie/arb-support/src/sandbox.rkt	148	0
(unnamed)	/home/nightlies/herbie/arb-support/src/sandbox.rkt	247	6
(unnamed)	/usr/share/racket/pkgs/profile-lib/main.rkt	40	10
profile-thunk	/usr/share/racket/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/usr/share/racket/collects/racket/engine.rkt	42	24