%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 11-1-7 on page 242.
%% 
%%  Copyright (C) 2009 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

\documentclass[]{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{194.59192pt}
\setlength\parindent{0pt}
\StartShownPreambleCommands
\DeclareMathVersion{foo}
\SetMathAlphabet{\mathbf}{foo}{OMS}{cmsy}{m}{n}
\DeclareMathAlphabet{\mathxy}{OT1}{cmr}{bx}{n}
\SetMathAlphabet{\mathxy}{foo}{OT1}{}{}{}
\StopShownPreambleCommands
\begin{document}
[$ \mathbf{y=f(x)}   $] [(bold)
\mathversion{bold}   $ \mathbf{y=f(x)} $] [(foo)
\mathversion{foo}    $ \mathbf{y=f(x)} $
                     $ \mathxy{y=f(x)} $] [(normal)
\mathversion{normal} $ \mathbf{y=f(x)} $]
\end{document}