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Regarding mathematical properties: in particular, closure properties.
The operations being closed under regular tree languages might be
desirable property..
I have seen two kinds of operations under which regular tree languages are
not closed.. that is the result is not a regular tree language..
(a) Consider the schema:
S -> s (A, S, B) | s ()
A -> a ()
B -> b ()
if we write a query like // (a | b) -- the result is (a^n b^n) which is
not regular tree language..
(b) Consider
S -> s (A*)
A -> a ()
consider the query
for $x in //a
return <b/>
for $x in //a
return <c/>
the result is (b^n c^n)
The question is has the new constructs in XPath made it not closed or
not.. It might have.. If someone has studied these aspects, then we can
know if XPath 2.0 is different from XPath 1.0 in these respects also..
cheers and regards - murali.
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