use-case.js

function LeaderBoard(doc) {
    var elem = Element(html)
        , playerSet = doc.createSet("type", "player")

    var adds = events(playerSet, "add")
        , players = map(adds, Player)
        , playerEls = map(players, prop("elem"))

    playerEls.pipe(append(elem))

    var selections = expand(players, function (player) {
            return events(player, "selected")
        })
        , selectedPlayer = reductions(to(), function (current, player) {
            if (current) {
                current.deselect()
            }

            player.select()
            return player
        }, null)

    selections.pipe(selectedPlayer)

    return {
        elem: elem
    }
}