Hal wrote:

I think this is where we are hurt by not having a VM model for policy evaluation --- or, in another view, not having a model for the relationship between walking the tree and executing nodal behaviors (here, policies with post-conditions). In other words, the operational lifecycle of the node...

In one view, there should be a prioritization of the evaluation of policies that hook nodal operations. This is extremely important, since a post-condition could cause the short-circuiting of parsing of the sub-tree under a particular node; one would therefore want higher-priority policies to evaluate before lower priority. Also, if certain policies set attributes that affect the outcome of other policies, one would want those to be of higher priority.

Some document processing models (e.g. Multivalent Documents [Phelps]) have a notion of "before" and "after" sequencing of nodal operations, with high priority behaviors having the "first and last words."