public function formElementName(string $var_name = ''): string
    {
        $parts = $var_name // This is optional.
            ? preg_split('/(\[[^[\]]*\])/u', $var_name, -1, PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY)
            : []; // An empty value could occur either way.

        if (!$parts) {
            return $this->data_var;
        } elseif (mb_strpos($parts[0], '[') === 0) {
            return $this->data_var.implode($parts);
        }
        return $this->data_var.'['.$parts[0].']'.implode(array_slice($parts, 1));
    }