I realize that comment wasn't clear.  What I was trying to express, which I'm only writing down now because I suspect it's worth moving away from his pattern going forward, is that it seems very hard to guess what reaching into numbered indices of call.on.args is likely to test.  I figured it out in another piece of code with this pattern today by setting a debugger breakpoint.

I'm not sure what the best way to make these sorts of tests more readable, but one possibility I can imagine is, instead of using anonymous functions of this stuff, expose them as _privateCallback specifically for testing purposes.  There might be better ways to do this, however.  Maybe even just firing something on the state machine directly?  CC @tOkeshu