Section isVoid.
Definition isVoid (sig : Type) (t : tape sig) :=
exists x rs, t = midtape rs x nil.
Definition isVoid_size (sig : Type) (t : tape sig) (s : nat) :=
exists x rs, t = midtape rs x nil /\ |rs| <= s.

Lemma mapTape_isVoid (sig tau : Type) (t : tape sig) (f : sig -> tau) :
isVoid (mapTape f t) <-> isVoid t.