bussproofs: \doubeLine missing

[jajaperson]

Is your feature request related to a problem? Please describe.
\doubleLine is a feature in true bussproofs which is missing in the MathJax version. This is used for writing bidirectional inference rules, e.g.

from

\begin{prooftree}
\def\fCenter{\vdash}
\Axiom$\Gamma \fCenter A$
\doubleLine
\UnaryInfC{$\Gamma, x:A \vdash$}
\end{prooftree}

Describe the solution you'd like
Implementation of \doubleLine for parity with true bussproofs.