data class Node(val value: String, val parents: List<Node> = emptyList(), var children: List<Node> = emptyList()){

    override fun toString() = value //avoid stack-overflow
    override fun hashCode() = value.hashCode()
    override fun equals(other: Any?) = value == (other as? Node)?.value
}

fun Node.bfs(): Sequence<Node> {
    val start = generateSequence(this) { it.parents.first() }.last()
    val queue: Queue<Node> = LinkedList<Node>().also { it.add(start) }
    val visited = HashSet<Node>()

    fun Node.hasUnvisitedParent() = (parents - visited).any()

    return buildSequence {
        while( ! queue.isEmpty()){

            while(queue.any() && (queue.peek() in visited || queue.peek().hasUnvisitedParent())){
                queue.remove()
            }

            if(queue.isEmpty()){ return@buildSequence }

            val next = queue.remove()

            yield(next)

            visited += next
            queue += next.children
        }
    }
}