That first response was all pretty irrational as I was just flummoxed and upset that you'd actually make that point to me. Yes, source filters can work. Apparently there's one that POE uses internally. I just resent the proliferation of known buggy techniques and would prefer to give no quarter to them. This happens to be something that's well known as impossible to get right 100% of the time. Maybe its actually pretty high now that stuff like Filter::Util exists. I dunno. I'll likely never reach for it as I don't know of any circumstance where it would be the right enough tool to allow introducing its quantum weirdness into my code.