Method begin

src/main/java/fieldbox/boxes/plugins/PadGroup.kt:96–107

        (q as MutableList<Box>).clear()
    }

    var beginCount: Int = 0

    fun begin() {
        if (beginCount == 0) {
            val q = parent.properties.computeIfAbsent(contents) { k -> ArrayList() }
            q.forEach { x -> x.properties.put(mark, false) }

            RunLoop.main.mainLoop.attach(Scene.perform(100, {
                end()
                false
            }))
            beginCount++
        }