fold koan

fold.kt

// Return the set of products that were ordered by every customer
fun Shop.getSetOfProductsOrderedByEveryCustomer(): Set<Product> {
    val allProducts = customers.flatMap { it.orders.flatMap { it.products } }.toSet()
    return customers.fold(allProducts) { acc, customer ->
    	acc.intersect(customer.orders.flatMap { it.products } ) 
	}.toSet()
}