Corrected entry: In the flashback that shows Adam Brenner playing his opponent using the word "Exvin", we can see that the X is also used in "Juxtapose". When the opponent adds an S to "Exvin", he is challenged and made to take back his tiles. However, the X would remain where it is because it is a part of another word. Since the opponent plays a word using an X later in the game ("Loxodrome"), we know that he must have taken back the X as well. However, that cannot be the case as then "Juxtapose" would not be able to stand without the X.