This is nice and clean, but it doesn't seem to catch
child::A/child::B/child::C.

I've been trying to work with a generalisation of your "single-level"
property:
a path expression has the "peer" property if no node in the set is an
ancestor of any other. Then (I think) P / Q is in document order if P is in
document order and P has the peer property and Q has the stays-in-subtree
property.

Saxon 4.7 implemented a rule that the result was ordered if all the steps
were downwards, which I now know to be incorrect.

Mike

> -----Original Message-----
> From: James Clark [mailto:jjc@xxxxxxxxxx]
> Sent: 28 October 1999 11:34
> To: xsl-list@xxxxxxxxxxxxxxxx
> Subject: Re: A theory problem
> 
> 
> A path expression has the property if 
> 
> (a) it doesn't use / and the axis is a forward axis, or
> 
> (b) it is a / expression, and the left hand operand has the
> "single-level" property and the right hand operand has the
> "stays-in-subtree" property.
> 
> A path expression E has the "single-level" property if and only if for
> any context node C, evaluating E wrt C yields a set of nodes all of
> which have the same parent.
> 
> A path expression E has the "stays-in-subtree" property if and only if
> for any context node C, evaluating E wrt C yields a set of 
> nodes all of
> which are in the subtree rooted at C (ie have C in their
> ancestor-or-self axis).
> 
> It's easy to see this is a sufficient condition for the path 
> expression
> to have the property: if x and y have the same parent, and x 
> is before y
> in document order, then any node in the subtree rooted at x is before
> any node in the subtree rooted at y.
> 
> XT implements this.
> 
> Kay Michael wrote:
> > 
> > 
> > A path expression returns a nodeset. There is a "natural order" to the
> > result which is obtained by following the steps in a particular
sequence.
> > For some path expressions the natural order will always be the same as
> > document order, for others it will not. Provide an algorithm that
examines a
> > path expression and determines whether it will always return nodes in
> > document order.


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