procedure decipher(cipher_txt, plain_alpha,
      cipher_alpha1, cipher_alpha2)
         local keys, values, digraphs, plain_txt

   values := create !plain_alpha || !plain_alpha
   keys := create !cipher_alpha1 || !cipher_alpha2

   digraphs := table()

   while digraphs[@keys] := @values

   plain_txt := ""

   cipher_txt ? {
      while plain_txt ||:= digraphs[move(2)]
      }

   return plain_txt

end