Hi Jukka,
Also the distinction between mutable and immutable Trees is pretty much the same as a between normal Maps and Collections.immutableMap(), so I don't think there's too much room for confusion. And allowing mutable Trees for specific cases like transient changes keeps the proposed API compact.
See https://gist.github.com/1934912 for what I meant about factoring mutability out for transient modifications into a decorator on top of a completely immutable (i.e. persistent) Tree.
Michael
