code: def append_op(self, operation):
          
          if operation not in self.ops:
              self.ops.append(operation)
          return self
docs: 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.