The function f(x) takes a word as input and applies a simple substitution cipher, where each letter in the input word is replaced by the letter three positions ahead of it in the alphabet, wrapping around to the beginning of the alphabet if necessary. For example, the input "Stonehenge" becomes "Evjatnse", where the first letter "S" is replaced by the third letter after it, "E", and the second letter "t" is replaced by the third letter after it, "j".

