def emit(self, ctx, modules, fd):
        """Main control function.
        """
        for (epos, etag, eargs) in ctx.errors:
            if error.is_error(error.err_level(etag)):
                raise error.EmitError("JTOX plugin needs a valid module")
        tree = {}
        mods = {}
        annots = {}
        for m,p in unique_prefixes(ctx).items():
            mods[m.i_modulename] = [p, m.search_one("namespace").arg]
        for module in modules:
            for ann in module.search(("ietf-yang-metadata", "annotation")):
                typ = ann.search_one("type")
                annots[module.arg + ":" + ann.arg] = (
                    "string" if typ is None else self.base_type(typ))
        for module in modules:
            self.process_children(module, tree, None)
        json.dump({"modules": mods, "tree": tree, "annotations": annots}, fd)
Main control function.