Converting if else to a when

TraverseWithWhen.kt

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

    fun extractText(e: Element): StringBuilder {
        when (e) {
            is Text -> sb.append(e.text)
            is Container -> for (child in e.children) {
                extractText(child)
            }
            else -> error("Unrecognized element : $e")
        }
        return sb
    }
    return extractText(this).toString()
}