code: def sign(self, wifkeys, chain=None):
          
          if not chain:
              chain = self.get_default_prefix()
          self.deriveDigest(chain)
  
          # Get Unique private keys
          self.privkeys = []
          for item in wifkeys:
              if item not in self.privkeys:
                  self.privkeys.append(item)
  
          # Sign the message with every private key given!
          sigs = []
          for wif in self.privkeys:
              signature = sign_message(self.message, wif)
              sigs.append(Signature(signature))
  
          self.data["signatures"] = Array(sigs)
          return self
docs: Sign the transaction with the provided private keys.
  
              :param array wifkeys: Array of wif keys
              :param str chain: identifier for the chain