Macro for sidenote.

[ascherer]

% put a mark in the left margin. (see Exercise 14.28 in The TeXbook)
\def\leftmark#1{\strut\vadjust{\vbox to 0pt
   {\vskip-\baselineskip\moveleft 1em\llap{\bf #1\strut}\vss}}\ignorespaces}

See file tex.ch from package tex-fpc. I've changed \hbox to llap and reduced the \moveleft to allow slightly larger notes.