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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 update(self, table, columns, values):
 """Update one or more existing table rows. :type table: str :param table: Name of the table to be modified. :type columns: list of str :param columns: Name of the table columns to be modified. :type values: list of lists :param values: Values to be modified. """

        self._mutations.append(Mutation(update=_make_write_pb(table, columns, values)))