Creating a box framework for inside the formula enviroment

Question

For a list of things to be used inside the formula enviroment, I want to create boxes stacked over each other. I basically want to write

$$ A := "Box with lines like 'A is a number', 'A is bigger than 3',... " $$

What I come up with is is

\newcommand{\mybox}[2]{\stackrel{\fbox{$#1$}}{\fbox{$#2$}}} 

but it pretty much sucks when it comes to what I really want. It's only two boxes and they don't even touch each other. Their length depends on the content. It should look more like a shelf. The length of all boxes should fit (the height might depend on the content). Also, the box should be oriented such that the := is in the vertical center. But if I want to use my construction for more than two boxes I have to do stupid things like

\mybox{statement 1}{\mybox{and statement 2}{statement 3}}

Answer 1

What about a simple tabular?

\documentclass{article}
\usepackage{colonequals}

\begin{document}

\begin{equation}
  A \colonequals
  \begin{tabular}{|c|}
    \hline
    statement 1\\\hline
    and statement 2\\\hline
    statement 3\\\hline
  \end{tabular}
\end{equation}

\end{document}

Result:

Edit: Making a n-ary function:

\documentclass{article}
\usepackage{colonequals}

\usepackage{kvsetkeys}

\makeatletter
\newcommand*{\boxes}[1]{%
  \begingroup
    \toks@={%
      \begin{tabular}{|c|}%
      \hline
    }%
    \comma@parse{#1}\box@entry
    \the\toks@
    \end{tabular}%
  \endgroup
}
\newcommand*{\box@entry}[1]{%
  \toks@=\expandafter{\the\toks@
    #1\tabularnewline
    \hline
  }%
}
\makeatother

\begin{document}

\begin{equation}
  A \colonequals
  \boxes{
    statement 1,
    and statement 2,
    {statement 3, 4 and 5},% comma needs to be protected by braces
    another statement,
  }
\end{equation}

\end{document}

Result:

The contents of the tabular is first written in a token register. That avoids trouble with cell groups and empty lines at the end.

Edit: Further reading:

• Token registers: Victor Eijkhout, TeX by Topic, Chapter 14: Token lists, CTAN:info/texbytopic/TeXbyTopic.pdf

• Package documentation of kvsetkeys, Section 1.7: Comma separated lists, CTAN:macros/latex/contrib/oberdiek/kvsetkeys.pdf