Safe ItemStack (see https://medium.com/@workingkills/nice-article-a67f57bc419f)

Example.kt

fun test() {
    val item1 = Item(GameObject())
    val item2 = Item(GameObject())
    assert(ItemStack(setOf(item1, item2)) is ItemStack)
    assert(ItemStack(setOf(item1)) is Item)
    assert(ItemStack(emptySet()) is Empty)
}

class ItemStack private constructor(val items: Set<Item>) : Elem() {
    companion object {
        operator fun invoke(items: Set<Item>): Elem =
            when (items.size) {
                0 -> Empty
                1 -> items.first()
                else -> ItemStack(items)
            }
    }

    // faking methods from data class
    fun copy(items: Set<Item> = this.items): Elem = Companion(items)
    override fun toString() = "${ItemStack::class.java.simpleName}(${this::items.name}=$items)"
    override fun equals(other: Any?) = other is ItemStack && other.items == items
    override fun hashCode() = items.hashCode()
    
    override fun plus(el: Elem): Elem =
        when (el) {
            is Empty -> this
            is Item -> ItemStack(this.items + el)
            is ItemStack -> ItemStack(this.items + el.items)
        }
}