func merge_streams(streams) {
    var s = streams.map { |stream|
        Pair(stream, stream.readline)
    }.grep {|p| defined(p.value) }

    gather {
        while (s) {
            var p = s.min_by { .value }
            take(p.value)
            p.value = (p.key.readline \\ s.delete_if { _ == p })
        }
    }
}

say merge_streams(ARGV.map {|f| File(f).open_r }).join("\n")