Explain how a statement works.

[bangerth]

I realized that this is not obvious after talking to Alberto Sartori.

[tjhei]

I find it a bit weird to explain how dynamic_cast and declaring variables inside an if scope work somewhere in the middle of some function, but I guess it doesn't hurt. :-)

Can you capitalize the beginning of sentences?

[bangerth]

It's not quite in the middle of a function but right in front of the block of places where we use this (not visible from the diff by itself). I thought that's probably the place to talk about it.

Re capitalization -- none of the other comments in the function do it. I don't think we're particularly consistent globally (though locally we are, using the style I used here). You tell me how you want it.