Definition dnorm (d:decimal) : decimal :=
  let norm_i i f :=
    match i with
    | Pos i => Pos (unorm i)
    | Neg i => match nzhead (app i f) with Nil => Pos zero | _ => Neg (unorm i) end
    end in
  match d with
  | Decimal i f => Decimal (norm_i i f) f
  | DecimalExp i f e =>
    match norm e with
    | Pos zero => Decimal (norm_i i f) f
    | e => DecimalExp (norm_i i f) f e
    end
  end.

Lemma dnorm_spec_f d : let f := match d with Decimal _ f => f | DecimalExp _ f _ => f end in let f' := match dnorm d with Decimal _ f => f | DecimalExp _ f _ => f end in f' = f.