while investigating in my grid layout problem I found a typo in the
documentation of multicol(.dtx):

l.533-537
% \subsection{Tracing the output}
%
% To understand the reasoning behind the decisions \TeX{} makes when
% processing a \mc{} environment, a tracing mechanism is provided.
% If you set the counter `\mc{}' to a positive \meta{number} you then

l.160
% \newcommand{\mc}{{\sf multicols}}

but obviously the counter should be tracingmulticols.

Ulrich Dirr