code: def derive_private_key(self, sequence):
          
          encoded = "%s %d" % (str(self), sequence)
          a = bytes(encoded, "ascii")
          s = hashlib.sha256(hashlib.sha512(a).digest()).digest()
          return PrivateKey(hexlify(s).decode("ascii"), prefix=self.pubkey.prefix)
docs: Derive new private key from this private key and an arbitrary
              sequence number