fn add_local_var_if_new(&mut self, id: IdentId) {
        if !self.is_local_var(id) {
            for c in self.context_stack.iter_mut().rev() {
                match c.kind {
                    ParseContextKind::For => {}
                    _ => {
                        c.lvar.insert(id);
                        return;
                    }
                };
            }
        }
    }
If the `id` does not exist in the scope chain,
add `id` as a local variable in the current context.
ruruby-parse/src/parser.rs