Moving function as a local func.

TraverseWithLocalFunc.kt

fun Element.extractText(): String {
    val sb = StringBuilder()

    fun extractText(e: Element): StringBuilder {
        if (e is Text) {
            val text = e
            sb.append(text.text)
        } else if (e is Container) {
            val container = e
            for (child in container.children) {
                extractText(child)
            }
        } else {
            error("Unrecognized element : $e")
        }
        return sb
    }

    return extractText(this).toString()

}