# mathtools with mu = Illegal unit?

I'm trying to change medmuskip while using mathtools:

\documentclass{article}
\usepackage{mathtools}
\setlength{\medmuskip}{0mu}
\begin{document}
\end{document}


But this results in an error:

! Illegal unit of measure (pt inserted).
m
l.3 \setlength{\medmuskip}{0mu}


Without mathtools, this works.

Why and what should I do to be able to change medmuskip?

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Probably not really helpful, but if I put \setlength{\medmuskip}{0mu} before \usepackage{mathtools}, it works for me. –  MaxD Jul 6 at 13:21

With the usual definition, \setlength{<parameter>}{<value>} is just a wrapper for

<parameter>=<value>\relax


so

\setlength{\medmuskip}{0mu}


is legal, because it translates to

\medmuskip=0mu\relax


which is perfectly legal, as \medmuskip should be assigned values only in mu units (also fil, fill or filll units for the plus and minus parts).

However, when calc is loaded (it's required by mathtools), \setlength changes meaning and mu units are not allowed any more, because calc is oriented to standard lengths only.

There's not much you can do about it other than defining a wrapper yourself:

\newcommand{\setmuskip}[2]{#1=#2\relax}


and do

\setmuskip{\medmuskip}{0mu}


or, say

\setmuskip{\medmuskip}{3mu plus 3mu}


(which is the default).

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Nice! So in this case, why not use only \medmuskip=0mu\relax? What is the advantage using a new command? –  Sigur Jul 6 at 14:02
@Sigur Uniformity of syntax. –  egreg Jul 6 at 14:02