Ultra-SiMPLE
coded = keyin text ("The coded message (upper case letters only): ")
decoded = ""
Do k = 0 To length(coded)-1
   letter = extract(coded,k,k)
   If (letter != " ")
      index = ascii(letter) - ascii("A")
      newindex = cycle(index-3,26)
      newascii = ascii("a") + newindex
      newletter = char(newascii)
   Endif
   Else newletter = " "
   decoded = decoded + newletter
Loop
Display "The decoded message is: ",decoded