$$\begin{prooftree}
\Gamma, x : A \prov M : B
\justifies
\Gamma \prov \lambda x . M : \Pi x : A . B
\end{prooftree}$$
rather quickly without even needing to hit the backspace key once, or really engaging in conscious thought at all. I'm not sure what this means. For one thing it certainly means I ought to have boilerplate stuff like this bound to some emacs key...
In other news, \usepackage{color} FTW! I now have a extremely compact way of displaying the two fairly parallel halves of my syntax. I can pretend I'm all cool like Phil Wadler.