code: def bip32_prv(self, s):
          
          data = self.parse_b58_hashed(s)
          if data is None or not data.startswith(self._bip32_prv_prefix):
              return None
          return self._network.keys.bip32_deserialize(data)
docs: Parse a bip32 private key from a text string ("xprv" type).
          Return a :class:`BIP32 <pycoin.key.BIP32Node.BIP32Node>` or None.