Bussproof with axiom labels

Question

How do I use \LeftLabel{} and \RightLabel{} along with just \AxiomC{}?

The following only displays the axiom

\documentclass[14pt]{extarticle}

\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath}
\usepackage{bussproofs}

\begin{document}
    \begin{prooftree}
        \LeftLabel{bar}
        \RightLabel{baz}
        \AxiomC{foo}
    \end{prooftree}
\end{document}

Answer 1

The package bussproofs needs an interference line (terminology by the package) which is generated with \UnaryInfC{some content} -- only after using that the labels from \RightLabel and \LeftLabel are shown.

\documentclass[14pt]{extarticle}

\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath}
\usepackage{bussproofs}

\begin{document}
    \begin{prooftree}
        \AxiomC{foo}
        \LeftLabel{bar}
        \RightLabel{baz}
        \UnaryInfC{Yo!}
        \singleLine
    \end{prooftree}
\end{document}

Answer 2

I don't think a bare axiom counts as a valid proof tree. I think you'd want something like

\documentclass[14pt]{extarticle}

\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath}
\usepackage{bussproofs}

\begin{document}
    \begin{prooftree}
        \AxiomC{}
        \LeftLabel{bar}
        \RightLabel{baz}
        \UnaryInfC{foo}
    \end{prooftree}
\end{document}

PDF docs suggest the labels are for inference lines, not bare axioms, so without an inference, there won't be any labels.