Overwrite \def to check if the command exists?

Question

LyX supports math-macros, but sadly when passed to the latex processor, they are defined by a \def rather than a newcommand.

I have found overwriting commands (e.g. using \def\v{\mathbf{v}}) to result in hard-to-debug errors, especially when BibTeX files entries introduce names, that require the original meaning of the command. Hence I'd like to change this behaviour -- asking for a change in LyX would however be unlikely to be successful (breaking backward compatibility) and at best require waiting for the next binary release (windows) or compiling it myself with a patch (linux).

Hence I was wondering if it is possible to (locally) overwrite \def to call \newcommand instead.

I tried

\documentclass[]{article}
\begin{document}

\begingroup % for locality of the redefinition
    \let\odef\def
    \renewcommand{\def}[1]{\newcommand{#1}{}\odef#1}
    \def\xx{{\bf x}} \xx
\endgroup

\end{document}

but this fails with

! TeX capacity exceeded, sorry [input stack size=5000].
\@ifstar #1->\@ifnextchar *{\@firstoftwo {#1
                                            }}
l.18  \def\xx
             {{\bf x}} \xx

presumably related to \newcommand using \def internally.

Answer 1

Figured it out myself already:

\documentclass[]{article}
\begin{document}

\let\odef\def
\odef\def#1{\odef\dummy{}{\let\def\odef\newcommand{#1}{}}\odef#1}
% <LYXDEFINITIONS>
\global\long\def\xx{{\bf x hello}} \xx
\global\long\def\yy{{\bf y world}} \yy 
% </LYXDEFINITIONS>
\let\def\odef

\end{document}

The trick was to return the original definition to \def for the duration of \newcommand.

Now only the question of undesirable side effects remains (though when used only for LyX's math macros there shouldn't be any).