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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 to_projection(self):
 """ Promote this column expression to a table projection """

        roots = self._root_tables()
        if len(roots) > 1:
            raise com.RelationError(
                'Cannot convert array expression '
                'involving multiple base table references '
                'to a projection'
            )

        table = TableExpr(roots[0])
        return table.projection([self])