Well, so, to continue, let's assume that there are no user-defined functions, and in fact the only thing we want to proof is select+filter, where a filter is limited to the default operators. From that is it follows that
-path1: select-child-nodes-by-name(select-child-nodes-by-name($context,'x'),'w') -path2: select-descendant-nodes-by-name($context,'w') And we need to proof that those functions are somehow equivalent. Can it be done? Op woensdag 27 januari 2016 heeft daniela florescu <[email protected]> het volgende geschreven: > > > > It seems to be a long-standing tradition that computer scientists, when > asked to prove a difficult conjecture C, respond by giving a proof for a > simplified conjecture C'. While this might lead to progress in the long > run, and enables them to get papers published in the academic literature, > it is totally useless to practical engineeers who want to know whether they > can safely rely on C. > > Michael, > > > what you say is nice and true. > > However given that: > 1. path expressions point (syntactically hence esemantically) into > XQuery’s expressions > 2. XQuery expression language is Turing complete > 3. Subsumption for a Turing complete language is undecidable. > > Well, I can hardly see a way to decide this problem other then by > introducing SOME restrictions > of some sort… but of course some restrictions that would not nullify the > original problem all together > and make the solution useless. > > Best, > Dana -- W.S. Hager Lagua Web Solutions http://lagua.nl
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