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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 Rminus_fp2 : forall r1 r2:R, frac_part r1 < frac_part r2 -> frac_part (r1 - r2) = frac_part r1 - frac_part r2 + 1.