Game of Life functional in Kotlin

GameOfLife.kt

package com.acme.jvm

import java.util.stream.Stream

typealias Board = List<List<Boolean>>

fun step(board: Board) = board.mapIndexed { rowIdx, row ->
    row.mapIndexed { colIdx, cellAlive ->
        neighboursAlive(board, rowIdx, colIdx) in if (cellAlive) 2..3 else 3..3
    }
}

fun neighboursAlive(board: Board, x: Int, y: Int) = neighbours(board, x, y).count { it }
fun neighbours(board: Board, x: Int, y: Int) = board.mapIndexed { rowIdx, row ->
    row.filterIndexed { colIdx, _ ->
        isNeighbour(rowIdx, colIdx, x, y)
    }
}.flatten()

fun isNeighbour(rowIdx: Int, colIdx: Int, x: Int, y: Int) =
        Math.abs(x - rowIdx) <= 1 && Math.abs(y - colIdx) <= 1 && (x != rowIdx || y != colIdx)

fun toMultiLineString(board: Board): String {
    val sb = StringBuilder()
    board.forEach{ row ->
        row.forEach{ column -> sb.append(if (column) "Ö " else "† ") }
        sb.append("\n")
    }
    return sb.toString()
}

fun main(args: Array<String>) {
    val board = listOf(listOf(false, true, false), listOf(false, true, true), listOf(true, false, false))
    Stream.iterate(board) { step(it) }
            .limit(100)
            .forEach { println(toMultiLineString(it)) }
}