Function tgamma1pm1_pair 

pub fn tgamma1pm1_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,

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

Boost calls the single-sign form tgamma1pm1. See the module docs for why tgamma(1 + v) - 1 is not an acceptable substitute.