"This is a matter where Pauan and I agree on the ideal semantics completely."
Quite the rare thing indeed.
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"I seem to remember us advocating egal a few other times (though I don't think I've ever called it "egal")."
I personally haven't. I only recently started to actually care about egal because of Nulan using immutable objects.
I do remember us having discussions before about making "iso" the default equality in Arc, which is what wart does, but I don't recall us talking about cons mutability.
However, I can't find any more than that. Maybe I just took it for granted at that point.
It is the policy I applied in Cairntaker. Cairntaker interns all immutable tables so they can be compared for deep equality even after some of their entries have been garbage-collected. Equality support is essential in Cairntaker since any value can be used as a table key.