Similar to https://twistedmatrix.com/trac/ticket/6538 , which is for tracing callback history on Deferreds, I think we should have this feature for Effects.

I hope that it will be easier for Effects. At the very least, we don't have as much compatibility baggage to maintain as Deferred, but I think the referential transparency of Effects, and the fact that there are known fixed points when execution of Effects starts and ends, should make it much easier.

However, there's still some difficulty in reporting these traces to the user when an error occurs. Right now, we only give users an exc_info tuple, which provides no space for this extra debugging info. This is something that should be considered when working on resolving #24.