private function lookupToken($str_val)
    {
        if('__key__' === $str_val) {
            return $str_val;
        }
        if('_' === $str_val[0]) {
            if(isset($this->arr_tokens[$str_val])) {
                return $this->arr_tokens[$str_val];
            }
        }
        if('@' === $str_val[0]) {
            $str_bind_name = substr($str_val, 1);
            if(isset($this->arr_named_params[$str_bind_name])) {
                return $this->arr_named_params[$str_bind_name];
            }
        }
        return $str_val;
    }