# How to place two proof trees next to each other with a symbol in between?

I have to typeset a transformation rule for proof trees, created with the bussproofs package. What I need is an implication sign between two proof trees - aligned with the horizontal separation lines of the trees - with exactly the same spacing to the left and right.

By changing the widths of the minipages and using a label in the first proof tree, I have more or less achieved the desired result, but it really is a kind of cheating.

Is there a durable solution?

\documentclass[border=10pt]{standalone}
\usepackage{amsmath,amssymb,bussproofs}
\newcommand{\necc}{\ensuremath{\mathbin{\Box}}}
\begin{document}
\begin{minipage}[b]{.3 \textwidth}
\begin{prooftree}
\AxiomC{$\phi_1\,\cdots\,\phi_n$}
\RightLabel{$\qquad\implies\qquad$}
\UnaryInfC{$\psi$}
\end{prooftree}
\end{minipage}
\begin{minipage}[b]{.25\textwidth}
\begin{prooftree}
\AxiomC{$\necc\phi_1\,\cdots\,\necc\phi_n$}
\UnaryInfC{$\necc\psi$}
\end{prooftree}
\end{minipage}
\end{document}


## 1 Answer

You can put in-line proof trees into a center environment:

\documentclass{article}% standalone doesn't support centering
\usepackage{amsmath,amssymb,bussproofs}
\newcommand{\necc}{\ensuremath{\mathbin{\Box}}}
\begin{document}
\begin{center}
\AxiomC{$\phi_1\,\cdots\,\phi_n$}
\RightLabel{$\qquad\implies\qquad$}
\UnaryInfC{$\psi$}
\DisplayProof
\AxiomC{$\necc\phi_1\,\cdots\,\necc\phi_n$}
\UnaryInfC{$\necc\psi$}
\DisplayProof
\end{center}
\end{document}


You can even use an equation environment:

\documentclass{article}
\usepackage{amsmath,amssymb,bussproofs}
\newcommand{\necc}{\ensuremath{\mathbin{\Box}}}
\begin{document}
$$\AxiomC{\phi_1\,\cdots\,\phi_n} \UnaryInfC{\psi} \DisplayProof \quad\implies\quad \AxiomC{\necc\phi_1\,\cdots\,\necc\phi_n} \UnaryInfC{\necc\psi} \DisplayProof$$
\end{document}