CustomElementClassLookup(self, fallback=None)
Element class lookup based on a subclass method.
You can inherit from this class and override the method:
lookup(self, type, doc, namespace, name)
to lookup the element class for a node. Arguments of the method:
* type: one of 'element', 'comment', 'PI', 'entity'
* doc: document that the node is in
* namespace: namespace URI of the node (or None for comments/PIs/entities)
* name: name of the element/entity, None for comments, target for PIs
If you return None from this method, the fallback will be called.