LaTeX array within inferrule from mathpartir

Array-In-InferRule.md

Sometimes I want to put an array inside of a mathpartir inferrule.

The straightforward thing doesn't work:

\begin{equation*}
\inferrule{Conclusion}{
  Premiss 1 \and
  \begin{array}{ll}
   1 & 2 \\  % note, this is where the problem happens
   3 & 4
  \end{array}
}
\end{equation*}

The problem is that \\ is defined by inferrule in an incompatible way from array.

Fortunately, the \\ inside an array env is just an alias for \@arraycr (see documentation for the array package (PDF)).

So we just need the following in the preamble:

\makeatletter % allow us to mention @-commands
\def\arcr{\@arraycr}
\makeatother

...
\begin{document}
  \begin{equation*}
  \inferrule{Conclusion}{
    Premiss 1 \and
    \begin{array}{ll}
      1 & 2 \arcr
      3 & 4
    \end{array}
  }
  \end{equation*}
\end{document}

That's all!