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(** * A synonym of [if] on [bool] *)
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Definition ifb (b1 b2 b3:bool) : bool :=
  match b1 with
    | true => b2
    | false => b3
  end.

Lemma negb_orb : forall b1 b2:bool, negb (b1 || b2) = negb b1 && negb b2.