fn check_delegate(&self) -> Result<(), ParseErr> {
        for ctx in self.context_stack.iter().rev() {
            if ctx.kind == ParseContextKind::Method {
                if ctx.lvar.delegate_param.is_some() {
                    return Ok(());
                } else {
                    break;
                }
            }
        }
        Err(error_unexpected(self.prev_loc(), "Unexpected ..."))
    }
Check whether parameter delegation exists or not in the method def of current context.
If not, return ParseErr.
ruruby-parse/src/parser.rs