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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add(self, cell):
 """Append a cell into the stack. Parameters cell : BaseRNNCell The cell to be appended. During unroll, previous cell's output (or raw inputs if no previous cell) is used as the input to this cell. """

        self._cells.append(cell)
        if self._override_cell_params:
            assert cell._own_params, \
                "Either specify params for SequentialRNNCell " \
                "or child cells, not both."
            cell.params._params.update(self.params._params)
        self.params._params.update(cell.params._params)