Excercise 4.3

test.ml

let translate = function 
	  'A' -> 'C' | 'B' -> 'A' | 'C' -> 'D' | 'D' -> 'B'
	  | _ -> invalid_arg "Not a plaintext string";;

let stringtail s =
	String.sub s 1 (String.length s - 1);;

let rec check s1 s2 = 
	match (s1, s2) with
	("", "") -> true
	| ("", _) -> false
	| (_, "") -> false
	| _ -> if translate s1.[0] = s2.[0] then
		check (stringtail s1) (stringtail s2)
	    else false;;