>>>>> "Frank" == Frank Shearar <frank.shearar at angband.za.org> writes:

Frank> There are some costs involved in this, of course: I create two
Frank> entire copies of the tree. One, making the mutable copy, seems
Frank> unavoidable: you cannot simply un-immutable the tree, because you
Frank> have no idea what other objects might have a reference to parts
Frank> of the tree, and they all assume the tree's immutable.

You appear not to be aware of #become:, or have already ruled it out for
reasons you haven't made clear.  Which is it?

-- 
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