Function carlson_rd 

pub fn carlson_rd<P, E, V>(x: V, y: V, z: V) -> V
where
    P: Policy,
    E: FloatElement + EllipticConsts,
    V: FloatVector<Element = E>,

Carlson symmetric integral R_D(x, y, z) = R_J(x, y, z, z) (degenerate third kind, used for the second-kind incomplete integral). Same duplication scheme as carlson_rf, plus an accumulated sum term.