This class sits in the validation pipeline, ready (when requested) to build subtrees of the validated
document for use in testing assertions against complex types. If no assertions are active, the class
passes input events through unchanged. If one or more assertions is active, it builds a tree corresponding
to the outermost active assertion. The nodes added to the tree will be typed. In the case of an element
node, the type annotation is added during the endElement call, after testing assertions against that element,
to satisfy the rule that when an assertion is evaluated, the target element has type AnyType but its attributes
and descendants are typed according to the appropriate schema declarations.