2

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?

Transformation

\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}
4

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}

demo


You can even use an equation environment:

\documentclass{article}
\usepackage{amsmath,amssymb,bussproofs}
\newcommand{\necc}{\ensuremath{\mathbin{\Box}}}
\begin{document}
  \begin{equation}
    \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{equation}
\end{document}

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