Admitted. (* QuickChick plus_comm *)
Fixpoint double (n:nat) :=
match n with
| O => O
| S n' => S (S (double n'))
end.

(* Specifications: *)
Lemma double_plus : forall n, double n = n + n .
Admitted. (* QuickChick double_plus. *)
Theorem evenb_S : forall n : nat,
evenb (S n) = negb (evenb n).