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(* Variable isolation tactic                     *)
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(* 
This module defines a few tactics :

   IsoleVar x H isolates one occurence of x in H 
   Warning it does not factorize, but it is still usefull.

   NormalizeRing H normalize the hypothesis H using ring.
   RewriteVar x H, isolates x in H and then substitutes x 
   everywhere.     
*)


Ltac NormalizeRing H :=
  match goal with
  | i:(?X1 = ?X2) |- _ =>
      match constr:(i) with
      | H => generalize H; ring_simplify X1 X2; clear H; intro H
      | _ => fail
      end
  end.

Lemma Lc : forall a b : F, a = b -> a - b = 0.