Traverse code with use forEach and method references

TraverseWithUseOfForEach.kt

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

    fun extractText(e: Element) {
        when (e) {
            is Text -> sb.append(e.text)
            is Container -> {
                e.children.forEach(::extractText)
            }
            else -> error("Unrecognized element : $e")
        }
    }

    extractText(this)
    return sb.toString()
}