Yeah, they said it doesn't work. (I mentioned that in the 5th post of this thread.)
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And now this... I guess my understanding wasn't that far off.
https://twitter.com/EliShffrn/status...41134599487488
I think the problem - such as it is - is more that development wasn't disciplined enough with the templating on Spark Double, so it works in a way that they didn't expect.'
As for fixing the issue with a rules change instead of errata, I don't think that the problem is "in rule 706" per se. There's a tangle here that involves stuff about copy effects which is in the layer rules, stuff that's in the rules for handling replacement effects, and the specific strangeness associated with "as ~ enters play" replacement effects.