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Message-ID:  <A0EC36FA-07DB-4D5B-B652-487C829D1D93@yahoo.de>
Date:         Fri, 22 Jan 2010 15:20:21 +0100
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              <LATEX-L@listserv.uni-heidelberg.de>
Sender: Mailing list for the LaTeX3 project <LATEX-L@listserv.uni-heidelberg.de>
From: Philipp Stephani <st_philipp@YAHOO.DE>
Subject: Collision between mathtools.sty and expl3
To: LATEX-L@listserv.uni-heidelberg.de
Precedence: list
Status: R

Hello,

mhsetup.sty (included by mathtools.sty) defines a few macros that expl3 wants to define anew, resulting in error messages.

\documentclass{minimal}
\usepackage{mathtools}% or mhsetup
\usepackage{expl3}% error
\begin{document}
\null
\end{document}

This results in the following errors:

% This is a LaTeX bug: check coding!
% ! Command name `\if_num:w' already defined! Current meaning: \ifnum.
% \msg_kernel_bug:x ...oding!}\tex_errmessage:D {#1}
%
% l.56 \cs_new_eq:NN \if_num:w
%                                        \tex_ifnum:D

% This is a LaTeX bug: check coding!
% ! Command name `\if_case:w' already defined! Current meaning: \ifcase.
% \msg_kernel_bug:x ...oding!}\tex_errmessage:D {#1}
%
% l.57 \cs_new_eq:NN \if_case:w
%                                        \tex_ifcase:D

% ! This is a LaTeX bug: check coding!
% ! Command name `\if_dim:w' already defined! Current meaning: \ifdim
%
% For immediate help type H <return>.
%  ...                                              
%
% l.169 \cs_new_eq:NN \if_dim:w
%                               \tex_ifdim:D

The macros \if_num:w, \if_dim:w, and \if_case:w are defined using \cs_new_eq:NN.  Maybe you want to change this into \cs_set_eq:NN, or fix mhsetup.sty accordingly.

Regards,
Philipp