Function threshold_sign 

pub fn threshold_sign<R>(
    config: &GenesisCeremonyConfig,
    public_key_package: &RootPublicKeyPackage,
    shares: BTreeMap<u16, RootKeyPackage>,
    message: &[u8],
    rng: &mut R,
) -> Result<RootSignature>
where
    R: RngCore + CryptoRng,

Create a FROST threshold signature from the exact predeclared signing set.