def append_op(self, operation):
        """Append an :class:`Operation <stellar_base.operation.Operation>` to
        the list of operations.

        Add the operation specified if it doesn't already exist in the list of
        operations of this :class:`Builder` instance.

        :param operation: The operation to append to the list of operations.
        :type operation: :class:`Operation`
        :return: This builder instance.

        """
        if operation not in self.ops:
            self.ops.append(operation)
        return self