proposition: Lemma almost_eq_tape_sym t1 t2 : almost_eq_tape t1 t2 -> almost_eq_tape t2 t1.
proof: Proof. move=> [] > /almost_eq_sym ? /almost_eq_sym ?. by constructor. Qed.
imports: Undecidability(TM.SBTM) PeanoNat Lia List ssreflect ssrbool ssrfun