What I’m trying to do
I’d like to align my proof tree using \fCenter, but I can’t make it work for more than 2 lines. As an example, the following code should render
$$
\begin{prooftree}
\Axiom$ \varphi \Rightarrow \fCenter \varphi$
\UnaryInf$ \varphi \wedge \psi \Rightarrow \fCenter\varphi$
\UnaryInf$ \Rightarrow \fCenter (\varphi \wedge \psi) \to \varphi$
\end{prooftree}
$$
but insted I am getting this:
As you can see it works fine on the first two lines, but any more and it stops working.
Things I have tried
MathJax documentation suggests it supports \fCenter but I can’t find any more information than that.