Environments with matched parens in \begin and \end

Question

Suppose I have this sort of expression in a document.

\multicolumn{8}{l}{%
\parbox{25em}{%
\begin{itemize}
\addtolength{\itemsep}{-1em}
\item foo
\item bar
\end{itemize}%
}}

I'd like to define a new environment, call it "comment" that would allow me to simple write

\begin{comment}
\item foo
\item bar
\end{comment}

and produce the same output. The problem I've encountered is that the parentheses that are opened at the start are only balanced at the very end. That creates a problem if I try to define the "begin"-section of the new environment in the flatfooted way:

\newenvironment{comment}{%
  \multicolumn{8}{l}{%
  \parbox{25em}{%
  \begin{itemize}
  \addtolength{\itemsep}{-1em}
}

The parentheses are unbalanced right now, and if I were to balance them inside the definition of the "begin" part of the environment, I'd get the wrong result.

Here's an MWE

    \documentclass{article}


\def\Boxes{%
  $\Box$ & $\Box$ & $\Box$ & $\Box$ & $\Box$ & $\Box$ & $\Box$%
}
\usepackage{fullpage}

\begin{document}

\begin{table}[h!]
  \begin{tabular}{l|ccccccc}
    & Tue & Wed & Thur & Fri & Sat & Sun & Mon\\
    \hspace{1em}B$^{\flat}$ Major & \Boxes\\

    \multicolumn{8}{l}{%
      \parbox{25em}{%
      \begin{itemize}
        \addtolength{\itemsep}{-1em}
      \item foo
      \item bar
\end{itemize}%
}}
\\
\end{tabular}
\end{table}
\end{document}

Answer 1

As noted in comments you can collect up the environment in lrbox. As noted in Werner's and Heiko's comments you can't use a standard \begin before \multicolumn so this fudges \begin slightly.

    \documentclass{article}
%! LaTeX Error: Command \Box not provided in base LaTeX2e.
% make supplied MWE processable, grumble
\usepackage{amssymb}

\def\Boxes{%
  $\Box$ & $\Box$ & $\Box$ & $\Box$ & $\Box$ & $\Box$ & $\Box$%
}
\usepackage{fullpage}

\makeatletter
% only need this for \begin{spanlist} syntax otherwise skip this and use
% \spanlist....\endspanlist
\let\oldbegin\begin
\let\oldend\end
\def\begin#1{%
  \expandafter\ifx\csname#1\endcsname\spanlist
    \expandafter\@firstoftwo
  \else
  \expandafter\expandafter\expandafter\oldbegin\expandafter\@gobble
  \fi
  \spanlist{#1}}
\def\end#1{%
  \expandafter\ifx\csname#1\endcsname\spanlist
    \expandafter\@firstoftwo
  \else
  \expandafter\expandafter\expandafter\oldend\expandafter\@gobble
  \fi
  \endspanlist{#1}}
% end of scary \begin/\end redefinition.
\makeatother

\newenvironment{spanlist}
 {\omit\begin{lrbox}{0}\begin{minipage}{25em}\begin{itemize}\addtolength{\itemsep}{-1em}}
 {\end{itemize}\end{minipage}\end{lrbox}%
  \span\multicolumn{7}{l}{\usebox{0}}}

\begin{document}

% never use[h] on its own
%\begin{table}[h!]
\begin{table}[htp!]
  \begin{tabular}{l|ccccccc}
    & Tue & Wed & Thur & Fri & Sat & Sun & Mon\\
    \hspace{1em}B$^{\flat}$ Major & \Boxes\\
\begin{spanlist}
      \item foo
      \item bar
\end{spanlist}
\\
\end{tabular}
\end{table}
\end{document}