code: def add_operation(self, operation):
          
          if operation not in self.operations:
              self.operations.append(operation)
docs: Add an :class:`Operation <stellar_base.operation.Operation>` to
          this transaction.
  
          This method will only add an operation if it is not already in the
          transaction's list of operations, i.e. every operation in the
          transaction should be unique.
  
          :param Operation operation: The operation to add to this transaction.