code: def sign(self, secret=None):
          
          keypair = self.keypair if not secret else Keypair.from_seed(secret)
          self.gen_te()
          self.te.sign(keypair)
docs: Sign the generated :class:`TransactionEnvelope
          <stellar_base.transaction_envelope.TransactionEnvelope>` from the list
          of this builder's operations.
  
          :param str secret: The secret seed to use if a key pair or secret was
              not provided when this class was originaly instantiated, or if
              another key is being utilized to sign the transaction envelope.