# array within frac, using nath package

I use the nath package to get automatically-sized delimiters, but it breaks having a \begin{array}{…}…\end{array} within a \frac{}{} (simply removing the \usepackage{nath} below fixes the example, but I need nath for other stuff).

\documentclass{article}
\usepackage{nath}
\begin{document}
$$\begin{array}{c}a\\b\\c\end{array}% This works \frac{numerator}{denominator}% This works % I want to put the array at the numerator's place \frac{\begin{array}{c}a\\b\\c\end{array}}{denominator}% This fails$$
\end{document}


I'm trying to do this to typeset some simple natural deduction proofs (one or several hypotheses followed by a horizontal line, then one or several conclusions), but packages like bussproofs.sty are clearly overkill (I don't need to typeset full proofs, just a single deduction rule). While \frac{}{} is semantically incorrect for this use, it is simple and gives the visual result I need.

Edit: The error message is ERROR: LaTeX Error: Environment ARRAY undefined.

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For some reason, nath uses \uppercase when working on fractions (I've not checked the details, but the error message is about ARRAY being undefined).

This hack seems to work:

\documentclass{article}
\usepackage{nath}
\newenvironment{ARRAY}[2][c]{\lowercase{\array[#1]{#2}}}{\endarray}

\begin{document}
$$\begin{array}{c}a\\b\\c\end{array}\quad \frac{numerator}{denominator}\quad \frac{\begin{array}{c}a\\b\\c\end{array}}{denominator}$$
\end{document}


A less hackish way is to hide array:

\newcommand{\morelines}[1]{\begin{array}{c}#1\end{array}}


and

$$\frac{\morelines{a\\b\\c}}{denominator}$$


or even

\newcommand{\morelines}[1]{\begin{array}{c}#1\end{array}}
\newcommand{\deduction}[2]{\frac{\morelines{#1}}{\morelines{#2}}}


$$\deduction{a\\b\\c}{denominator}$$


I'm more convinced about not using nath.

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@GeorgesDupéron I've added a less hackish method –  egreg Jun 17 '12 at 15:21
A documentation with $$...$$ gives the result -- I am too old. –  Marco Daniel Jun 17 '12 at 16:05
@MarcoDaniel : What do you mean ? I don't get it. Besides, thanks for the update egreg :) . –  Georges Dupéron Jun 17 '12 at 18:56
@GeorgesDupéron: The documentation use $$...$$ for there examples. However this isn't correct. –  Marco Daniel Jun 17 '12 at 19:38
@MarcoDaniel I don't know why  is used in the documentation. I'm more concerned about using nath in general: it takes too much for granted. For instance, a formula like the one on page 4 (A/B\otimes C/D) would rarely be written with full fractions in display mode, in my field. –  egreg Jun 17 '12 at 19:44

After some more trial and error, I found that \hline works in the array environment.

\documentclass{article}
\usepackage{nath}
\begin{document}
$$\begin{array}{c} Hypothesis_1\\ Hypothesis_2\\ \hline Conclusion_1 \end{array}$$
\end{document}


This gives the visual result I want, and doesn't have \frac{}{}'s semantic mismatch, however the arrays have a different vertical alignment, and tend to collide with what's left and right of them (they need a little more horizontal padding), so egreg's answer is better.

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For the particular example in the question, a \matrix could be used as well
\documentclass{article}
$$\frac{\matrix a\\ b\\ c\strut\endmatrix}{denominator}$$
`