-of a multitape machine, and an additional one to mark the position of the head.
-We extend the tape alphabet with two new symbols true and false.
-false is used to pad shorter tapes, and true to mark the head of the tape *)
+of a multitape machine.
+We extend the tape alphabet with a special symbol None to pad shorter tapes.
+For the moment, the actual content of the tape alphabet is left unspecifed,
+but we shall need characters to store states and moves of the multitape
+machines and to mark the head position *)