private Entry lvalue_id(String id, int lno) {
        Entry e = symtab.search(id);
        if( e == null) {
            System.err.println("undeclared variable "+ id + " on line "
                               + lno);
            System.exit(1);
        }
        if( !e.isVar()) {
            System.err.println("constant on left-hand-side of assignment "+ id + " on line "
                               + lno);
            System.exit(1);
        }
        if( e.getIsIV()) {
            System.err.println("index variable on left-hand-side of assignment "+ id + " on line "
                               + lno);
            System.exit(1);
        }
        gcprintid(id);
        return e;
    }