code: def to_curve25519_private_key(self):
          
          sk = self._signing_key
          raw_private = nacl.bindings.crypto_sign_ed25519_sk_to_curve25519(sk)
          return _Curve25519_PrivateKey(raw_private)
docs: Converts a :class:`~nacl.signing.SigningKey` to a
          :class:`~nacl.public.PrivateKey`
  
          :rtype: :class:`~nacl.public.PrivateKey`