public JCClassDecl ClassDef(JCModifiers mods,
                                Name name,
                                List<JCTypeParameter> typarams,
                                JCTree extending,
                                List<JCExpression> implementing,
                                List<JCTree> defs)
    {
        JCClassDecl tree = new JCClassDecl(mods,
                                     name,
                                     typarams,
                                     extending,
                                     implementing,
                                     defs,
                                     null);
        tree.pos = pos;
        return tree;
    }