fun filter(lines: List<List<Int>>, fn: KFunction1<List<Int>, Int>): List<Int> {
    var index = 0
    var candidates = lines
    while (candidates.size > 1) {
        candidates = candidates.filter { line -> line[index] == fn(candidates.column(index)) }
        index++
    }
    return candidates.first()
}

fun filter(lines: List<List<Int>>, fn: KFunction1<List<Int>, Int>): List<Int> =
    lines.indices.fold(lines) { candidates, index ->
        if (candidates.size == 1) candidates
        else candidates.filter { line -> line[index] == fn(candidates.column(index)) } }.single()