array within frac, using nath package

Question

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{equation}
  \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{equation}
\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.

Answer 1

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

---

A less hackish way is to hide array:

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

and

\begin{equation}
\frac{\morelines{a\\b\\c}}{denominator}
\end{equation}

or even

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

and inputting your deduction as

\begin{equation}
\deduction{a\\b\\c}{denominator}
\end{equation}

---

_{I'm more convinced about not using nath.}

Answer 2

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

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

Answer 3

For the particular example in the question, a \matrix could be used as well

\documentclass{article}
\usepackage{nath}
\begin{document}
\begin{equation}
  \frac{\matrix a\\ b\\ c\strut\endmatrix}{`denominator}
\end{equation}
\end{document}