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 tape_contains_size_contains t x s :
tape_contains_size t x s -> tape_contains t x.