Turn off locally one package

In the following code, I would like to show two different formatting styles: one using the package nath output, and another showing the default output.

How can I do that?

% Source : http://tex.stackexchange.com/questions/1023/vertically-asymmetric-size-variation-for-parentheses

\documentclass[10pt,a4paper]{article}
\usepackage[utf8x]{inputenc}
\usepackage{ucs}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\usepackage{nath} % The usefull package.

\newcommand{\formulaOne}{\sum_{0 \le i \le m \\ 0 < j < n} P(i,j)}
\newcommand{\formulaTwo}{1 + \frac{1 + x^2}{1 - \frac{x}{2}}}

\begin{document}

\section{With nath}

$$( \formulaOne ) = [ \formulaTwo ]$$

\section{Without nath}

$$\left( \formulaOne \right) = \left[ \formulaTwo \right]$$

\end{document}

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this works:

\documentclass[10pt,a4paper]{article}
\usepackage{amsmath}
\begin{document}

\section{Without nath}
$$\left( \sum_{0 \le i \le m \\ 0 < j < n} P(i,j) \right) = \left[ 1 + \frac{1 + x^2}{1 - \frac{x}{2}} \right]$$

\section{With nath}

\makeatletter
\input{nath.sty}
\makeatother
$$( \sum_{0 \le i \le m \\ 0 < j < n} P(i,j) ) = [ 1 + \frac{1 + x^2}{1 - \frac{x}{2}} ]$$

\end{document}

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The nath package changes too much of the default setting that such a project, even if it was feasible (I don't think so, though), would require a gigantic effort.

The best strategy is to prepare the samples with the standalone class and to import them as PDF graphics.

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Thanks for your comment. –  projetmbc Oct 20 '11 at 13:33
While Herbert's answer apparently works in this instance, I think this answer is the more generally applicable... –  Seamus Oct 20 '11 at 14:08