Moreover, the application can have access to the transformational relationship between the type associated with a node's actual tag and the type that led to the validation of the node's children. The heads of those productions are the names of types. The types are associated with attribute definitions. There is a partial order on types that maximizes refinable content models. Instances are ordered, labeled, attributed trees.
Basic validity is defined in terms of types that are maximal in the partial order.
Validity with derivation is defined relative to lesser types in the partial order. The partial order on types is segmented in such a way as to mimic various classes of derivation. Validity is further sharpened to allow for tolerance of instances according to whether they fall into certain classes of derivation or not. A type is basic if there does not exist a distinct type such that . Also, we will sometimes call the signature of .