1. An extensible markup language (XML) path (XPATH) expression debugging method comprising: receiving an XPATH input expression; parsing the XPATH input expression to produce a plurality of sub-expressions; ordering the sub-expressions in a model for the XPATH input expression by associating each sub-expression with a node in the model; responsive to a selection of one of the sub-expressions in the model, producing a result set for the sub-expression by computing a result set for a sub-expression associated with a selected node in the model in a user interface; and if the result set does not isolate a point of failure in the XPATH input expression, recursively selecting another sub-expression and computing another result set for said another sub-expression until a point of failure in the XPATH input expression is isolated.