8. The system of claim 6 , wherein the XTT model comprises: a full XPATH expression node for the XPATH input expression; and, a plurality of sub-nodes coupled to the full XPATH expression node in a tree hierarchy, selected ones of the sub-nodes comprising an association with one of a step node, a function node and a predicate node.
9. The system of claim 8 , wherein the step node comprises a parenthesis node.