> but allows clobbering immutable data

The first thing `appender` does is look at the capacity of the passed in array, to see if it can append to it in-place or not. If it clobbers *anything*, immutable or not immutable, then it's a bug.

The call *should* be safe because of that. The fact that it is clobbering this array at all seems to imply a bug in the implementation. I know appender currently writes "past the un-used capacity without marking it as used",  but I've never heard of it "writing to used data".

I'm worried. This is supposed to be text-book example of how to use appender.