when would a frozenset be needed?

[Skaperen]

i'm not saying (the opposite of) that. i'm saying... modifications to FrozenMapCopy ...do not actually change that FrozenMapCopy (but instead record the modifications in a way that allows a new frozenmap created from it to be as if the frozenmap were mutable and copied). i get this from that example code (the big example after frozenmap.mutating()) that did del copy[591221] twice without an error.