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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
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(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
by move=> x y f; rewrite [_.+1](addnC x.+1).
Qed.
by move=> x y f; rewrite {2}[in f _]addnC.
Qed.
by move=> x y f; rewrite [in f _](addnC y).
Qed.

Lemma test2'' : forall x y f, f (y + x) + f(y + x) + f(y + x)  = f(x + y) + f(y + x) + f(x + y).