fun main() {
    val testCase = readln().toInt()
    for (case in 1 .. testCase) {
        val wordsCount = readln().toInt()
        val words = readln().split(' ')
        when(val result = solve(wordsCount, words)) {
            null -> println("Case #$case: IMPOSSIBLE")
            else -> {
                println("Case #$case: POSSIBLE")
                println(result.joinToString(" "))
            }
        }
    }
}

fun solve(wordsCount: Int, words: List<String>): List<String>? {
    val result = mutableListOf(words.first().toList().sorted().joinToString(""))
    for (i in 1 until wordsCount) {
        val next = words[i]
        result.add(next.makeGe(result.last()) ?: return null)
    }
    return result
}
fun String.nextGreater(): String? {
    var last = lastIndex - 1
    while (last >= 0 && this[last] >= this[last + 1]) {
        --last
    }
    if (last < 0) return null
    var next = last + 1
    while (next < length && this[last] < this[next]) {
        ++next
    }
    --next
    val result = this.toCharArray()
    result[next] = this[last]
    result[last] = this[next]
    return result.take(last + 1).joinToString("") + result.drop(last + 1).reversed().joinToString("")
}
fun String.makeGe(target: String): String? {
    val count = IntArray(26){0}
    for (c in this){
        count[c - 'A'] += 1
    }
    val result = StringBuilder()
    var last = 0
    while (last < target.length) {
        val c = target[last] - 'A'
        if (count[c] == 0) break
        result.append(target[last])
        --count[c]
        ++last
    }
    if (last == target.length) {
        for (i in count.indices) {
            while (count[i] > 0) {
                --count[i]
                result.append('A' + i)
            }
        }
        return result.toString()
    }else {
        val c = target[last] - 'A'
        return when(val next = (c until 26).find { count[it] > 0 }) {
            null -> {
                for (i in count.indices.reversed()) {
                    while (count[i] > 0) {
                        --count[i]
                        result.append('A' + i)
                    }
                }
                result.toString().nextGreater()
            }
            else ->  {
                result.append('A' + next)
                --count[next]
                for (i in count.indices) {
                    while(count[i] > 0) {
                        --count[i]
                        result.append('A' + i)
                    }
                }
                result.toString()
            }
        }
    }
}