Create the top-level structure for module `yam`.

def create_roots(self, yam):
        """Create the top-level structure for module `yam`."""
        self.local_grammar = SchemaNode("grammar")
        self.local_grammar.attr = {
            "ns": yam.search_one("namespace").arg,
            "nma:module": self.module.arg}
        src_text = "YANG module '%s'" % yam.arg
        revs = yam.search("revision")
        if len(revs) > 0:
            src_text += " revision %s" % self.current_revision(revs)
        self.dc_element(self.local_grammar, "source", src_text)
        start = SchemaNode("start", self.local_grammar)
        self.data = SchemaNode("nma:data", start, interleave=True)
        self.data.occur = 2
        self.rpcs = SchemaNode("nma:rpcs", start, interleave=False)
        self.notifications = SchemaNode("nma:notifications", start,
                                        interleave=False)