Function lgamma1p_pair 

pub fn lgamma1p_pair<P, E, V, const NE: usize, const NO: usize>(
    v: V,
    t: &LogGamma1p<E, NE, NO>,
) -> (V, V)
where
    E: FloatElement,
    V: FloatVector<Element = E> + TranscendentalMathWithPolicy,
    P: Policy,

$(\ln\Gamma(1+v),\; \ln\Gamma(1-v))$ for $\lvert v\rvert \le 1/2$.

Outside that range the series has not been given enough terms and the result is wrong. The caller reduces the order first. Exact at $v = 0$, where both halves are zero.