Difficulty: 9-10 - Probably uses recursion - This is the one I want
to discuss in class

Substitution Cipher

(file name: cipher.c, cipher.p, cipher.C)

Antique Comedians of Malidinesia would like to play a new discovered comedy
of Aristofanes.
Putting it on a stage should be a big surprise for the audience so all the
preparations must be kept absolutely secret. The ACM director suspects one
of his competitors of reading his correspondece. To prevent other companies
from revealing his secret, he decided to use a substitution cipher in all
the letters mentioning the new play.

Substitution cipher is defined by a substitution table assigning each
character of the substitution alphabet another character of the same
alphabet. The assignment is a bijection (to each character exactly one
character is assigned -- not neccessary different). The director is afraid
of disclosing the substitution table and therefore he changes it frequently.
After each change he chooses a few words from a dictionary by random,
encrypts them and sends them together with an encrypted message. The plain
(i.e. non-encrypted) words are sent by a secure channel, not by mail. The
recipient of the message can then compare plain and encrypted words and
create a new substitution table.

Unfortunately, one of the ACM cipher specialists have found that this system
is sometimes insecure. Some messages can be decrypted by the rival company
even without knowing the plain words. The reason is that when the director
chooses the words from the dictionary and encrypts them, he never changes
their order (the words in the dictionary are lexicographically sorted).
String a1a2 ... ap is lexicografically smaller than b1b2 ... bq if there
exists an integer i, i <= p, i <= q, such that aj=bj for each j, 1 <= j < i
and ai < bi.

The director is interested in which of his messages could be read by the
rival company. You are to write a program to determine that.

Input Specification

The input consists of N cases. The first line of the input contains only
positive integer N. Then follow the cases. The first line of each case
contains only two positive integers A, 1 <= A <= 26, and K, separated by
space. A determines the size of the substitution alphabet (the substitution
alphabet consists of the first A lowercase letters of the english alphabet
(a--z) and K is the number of encrypted words. The plain words contain only
the letters of the substitution alphabet. The plain message can contain any
symbol, but only the letters of the substitution alphabet are encrypted.
Then follow K lines, each containing exactly one encrypted word. At the next
line is encrypted message.

Output Specification

For each case, print exactly one line. If it is possible to decrypt the
message uniquely, print the decrypted message. Otherwise, print the sentence
'Message cannot be decrypted.'.

Sample Input

2
5 6
cebdbac
cac
ecd
dca
aba
bac
cedab
4 4
cca
cad
aac
bca
bdac

Output for the Sample Input

abcde
Message cannot be decrypted.