Is it possible to make a macro sensitive to \\?

Question

I have the following macro:

   \newcommand{\INFERRULE}[3][]{%
   \ensuremath{\inferrule{\mathstrut #2}{\mathstrut #3}\ifx\\#1\\\else\enskip(#1)\fi}}

I want to make it sensitive to \\ so that I can write commands such as:

   \[
   \INFERRULE[i \to j]{[i, i, j, k] \\ [i, k, j, j]}{[i, i, k, j]}
   \]

However, the way I defined the macro, it is ignoring the \\. Is there a way to fix it?

Answer 1

I'm not sure if you're still interested in an answer, but assuming the following things about your question:

• You are using the mathpartir package which provides the \inferrule macro

• You would like to have multi-line premises

it seems easy to do. The mathpartir documentation says

A newline can be forced by adding an empty line \\\\

\inferrule {aa \\\\ bb} {dd \\ ee \\ ff}

Since your \INFERRULE is a just a wrap-around for \inferrule you can do the same thing. Here is a working example:

\documentclass{article}
\usepackage{mathpartir}
\newcommand{\INFERRULE}[3][]{%
\ensuremath{\inferrule{\mathstrut #2}{\mathstrut #3}\ifx\\#1\\\else\enskip(#1)\fi}}
\begin{document}
\[
   \INFERRULE[i \to j]
        {[i, i, j, k] \\ [i, k, j, j]}
        {[i, i, k, j]}
\]
\[
    \INFERRULE[i \to j]
        {[i, i, j, k] \\\\ [i, k, j, j]}
        {[i, i, k, j]}
\]    
\end{document}