Create top-level elements of the hybrid schema.

def setup_top(self):
        """Create top-level elements of the hybrid schema."""
        self.top_grammar = SchemaNode("grammar")
        self.top_grammar.attr = {
            "xmlns": "http://relaxng.org/ns/structure/1.0",
            "datatypeLibrary": "http://www.w3.org/2001/XMLSchema-datatypes"}
        self.tree = SchemaNode("start")