[texhax] fbox in align environment

[Philip G. Ratcliffe]

>    \usepackage{amsmath}
> 
>    \begin{equation*}
>      \setlength{\arraycolsep}{0pt}
>      \renewcommand{\arraystretch}{1.25}
>      \begin{array}{r@{\;}c@{\;}r@{\;}}
>        x+y+2    & = & 1\\
>        2x+3y-4z & = & 2\\
>        \hline
>        \multicolumn{1}{|r@{\;}}{3x-2y+z}
>        & \multicolumn{1}{c@{\;}}{=}
>        & \multicolumn{1}{r@{\;}|}{3}\\
>        \hline
>      \end{array}
>    \end{equation*}

A couple of comments:

1. the alignment of the third column ought to be with "l" (not "r");
2. if, e.g., the r.h.s. of one of the first two equations is longer (say,
1+p) then the output looks a little odd since the box is always as wide as
the table and not a given entry (the same is also true w.r.t. the l.h.s.).

Cheers,  Phil