Add comments to example solution

exercises/practice/isogram/.meta/example.sml

@@ -1,8 +1,19 @@
 val isIsogram: string -> bool =
   let
     val a = Char.ord #"a"
+
+    (* We represent each letter by setting a different bit,
+     * for example the letter c is encoded as ...00100 binary.
+     *)
     fun letterValue (c: char): word =
       Word.<<(0wx1, Word.fromInt(Char.ord (Char.toLower c) - a))
+
+    (* seen represents the set of letters that have been seen
+     * so far. For example, if we have seen the letters d and c,
+     * this is encoded as ...01100 binary.
+     * If we find a letter in l that matches a letter already
+     * seen, we return false.
+     *)
     fun recurse (seen: word) (l: char list): bool =
       case l of
           nil => true