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!

This is great! I don't want to admit to the nasty things I've done in the past to hack my way through aligning things in different rows using mathpartir.

another solution indicated by the manual is to wrap the whole array in braces { \begin{array} ... \end{array}}

Thank you, saved my life.