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(** *    Fractional part                                 *)
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Definition Int_part (r:R) : Z := (up r - 1)%Z.(**********)
Definition frac_part (r:R) : R := r - IZR (Int_part r).

Lemma fp_nat : forall r:R, frac_part r = 0 -> exists c : Z, r = IZR c.