Expressive power of XQuery node construction

In the relational model it has been shown that the flat relational algebra has the same expressive power as the nested relational algebra, as far as queries over flat relations and with flat results are concerned [11]. Hence, for each query that uses the nested relational model and that, with a flat table as input always has a flat table as output, there exists an equivalent flat query that only uses the flat relational model. In [12] a very direct proof is given of this fact using a simulation technique. In analogy, we study a related flat-flat problem for XQuery. We show that for each expression that only has original, copied or equal nodes in its result sequence there exists an expression without node construction yielding the same or a deep-equal result. In this work we will show how to generate automatically equivalent constructor-free expressions for node-conservative expressions. This result gives an indication of the expressive power of the node construction.