fun ArrayList<HashSet<String>>.union() : HashSet<String> {
    val result = this.fold  (HashSet<String>()) {
        left, right ->
        left.addAll(right)
        left
    }
    return result
}

fun ArrayList<HashSet<String>>.intersection() : HashSet<String> {
    val result = this.fold (this.union()) {
        left,right ->
        left.intersect(right).toHashSet()
    }
    return result
}