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(* 02110-1301 USA                                                     *)
(********************************************************************** Proof of Huffman algorithm: sTactic.v  Useful tactics  Tactics: Contradict, CaseEq, ElimEq Laurent.Thery@inria.fr (2003) **********************************************************************)

(* Specifications: *)
Theorem Contradict1 : forall a b : Prop, b -> (a -> ~ b) -> ~ a.
Theorem Contradict2 : forall a b : Prop, b -> ~ b -> a.
Theorem Contradict3 : forall a : Prop, a -> ~ ~ a.
Ltac Contradict name :=
(apply (fun a : Prop => Contradict1 a _ name); clear name; intros name) ||
(apply (fun a : Prop => Contradict2 a _ name); clear name);
try apply Contradict3.