Is there a way to limit the scope of \usepackage?

Question

When I tried to use the breqn package suggested by an answer to the question

How do I break a long equation over lines

It managed to break a whole bunch of unrelated equations (equations that did not use its functionality), including a definition for the control sequence \mf which I had previously defined myself.

Is there a way to limit the scope of a \usepackage declaration so that its (positive and negative) effects are limited to a particular segment of a document?

I have tried putting it inside an environment, but get the following error:

! LaTeX Error: Can be used only in preamble.

Answer 1

Perhaps for some small package this could be possible, but the general answer is no. In particular, breqn and its companion package flexisym do very extensive changes to the normal setup of math formulas which are, in some cases, global; therefore, even if it were possible to load them in a group, the changes would be performed also at the outer level. The same is true for many other packages.

It's not to be neglected that many commands that are used in packages are disabled after \begin{document} and this makes quite difficult to cope with the task of loading a package after the document has begun.

Answer 2

If instead of using \usepackage you use \input, you can limited the scope of the package. It will require redefining some commands and patching others, but it should be technically possible. For example

\documentclass{article}
\begin{document}
  \begingroup
    \makeatletter
      \renewcommand{\usepackage}[1]{\input{#1}}
      \renewcommand{\NeedsTeXFormat}[1]{}
      \renewcommand{\ProvidesPackage}[1]{}
      \usepackage{fixltx2e.sty}
    \makeatother
  \endgroup
\end{document}