A description for a simple definition of NVX

If we simply treat an XML document to be a tree without considering any features introduced by XML itself, each node could be taken as a 3-tuple (id, value, pointer, constraints).

1. id: the identifier of the very node; it should be unique to each node, such as a number, a id string etc. Usually, it should not be null value, but possibly be.
1. When id is a meaningful identifier, it is corresponded to an existing node in the XML document;
2. When id belongs to one sort of null values, the situation is a little complicated:
1. it must be something. None is not supposed to be permitted here.
2. it could be a mark, representing a set of nodes satisfying some conditions.
2. value: the content hold in the very node; it's kind of tag-related information. Possibly, the value could be an integer, a real number, a string etc. The tag-related Null values could happen in the place.
1. The definition of tag-related Null values could exploit those in Selcuk's report. Such marks as ex_mar, ma_mar, pl_mar, pa_mar, pm_mar can be used to represent null values here.
3. pointer: structure-related information about the very node, that is, it points nodes directly connected with the concerned node. In reality, we put the ids of those connected nodes here. The structure-related Null values could happen here.
1. Usually, the pointer should point one node. In such a case, the only exception is when the concerned node is root of the whole XML document.
2. A mark for one kind of null values is also possible here.
1. it could be null, which means the node linked doesn't exist.
2. it could be a mark for a set of nodes which satisfies a certain of condition.
4. constraints: this item gives an 'explanation' on the Null values used for this node.

Next, a more formal representation of the node in the XML document would be present.