indexing fails in svmono

Question

Buildign the code below (pdflatex, makeindex, pdflatex) causes an error

! LaTeX Error: Environment multicols undefined.

Code:

\documentclass{svmono}
\usepackage{makeidx}
\makeindex
\begin{document}

\frontmatter
\mainmatter
\index{foo}Text

\backmatter
\printindex

\end{document}

I can of course make the error go away using \usepackage{multicol} (and shall note this in an answer below), but I'd like to understand what's happening: has svmono simply failed to declare a dependency?

Answer 1

svmono.cls contains the lines

\if@threecolind
                   \begin{multicols}{3}[\chapter*{\indexname}%
                \else
                   \begin{multicols}{2}[\chapter*{\indexname}%
                \fi

in the "begin" part of the definition of the index environment. As you can see, the index will be built using the multicols environment and this requires the multicol package.

The documentation for svmono mentions that both the makeidx and multicol packages are required to build an index, so you need to load both packages.