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 isVoid_right (sig : Type) (t : tape sig) :
isVoid t -> right t = nil.