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Re: Constraints on the elements in a nested XML tree?

  • From: David Carlisle <d.p.carlisle@gmail.com>
  • To: "Costello, Roger L." <costello@mitre.org>
  • Date: Tue, 23 Jan 2018 23:10:00 +0000

Re:  Constraints on the elements in a nested XML tree?

On 23 January 2018 at 22:54, Costello, Roger L. <costello@mitre.org> wrote:
Hi Folks,

I am dealing with XML trees.

No id/idrefs. No mixed content.

Just plain old properly nested elements.

Suppose I have a pile of elements and I want a machine to assemble them into a tree of properly nested elements. The machine doesn't know the rules for assembly so I must tell it the constraints on the elements; in particular, the constraints on how they may be nested.

Here's the list of constraints that I've come up with thus far:

1. An element cannot be a descendent of itself, i.e., no loops in the tree.

2. If elements e and e' are children of element E, then e cannot be a child of e' or vice versa.

What other constraints are there?



You have missed the main defining properties of a tree

each node only has one parent (this one means you don't need your second constraint)
the tree is connected with a unique ancestor

But the constraints can (mostly) only validate the result, you are going to need a lot more than that to specify how to assemble
and that assembly specification will presumably produce a tree without needing these constraints?
that is

if you have a b c  then the possible trees are


then the 6 permutations of a,b,c so 12 possibilities

But any specification which tells your machine which of the 12 possibilities to build probably doesn't need to check the constraints if it just arranges to always build a tree,.

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