<p>I thought about this for a while.  Trees have generally been passed as pointers, in part because internally trees must hold pointers to their children.  But, I suppose, it is not necessarily reasonable to require that trees be passed as pointers.  So, I can't justify any reason why we shouldn't make this change.  But, there are a couple things we have to consider:</p>

<ul>
<li><p>What if the user prefers to pass a tree that should be copied?  Maybe we should provide overloads with just a reference instead of an rvalue reference?</p></li>
<li><p>We have to maintain reverse compatibility, so unfortunately we need to retain the overloads that take <code>TreeType*</code>.  We can just have those call the reference/rvalue reference overloads, though.  I think that in general they should call the rvalue-reference overloads since those functions generally took ownership of the tree.</p></li>
</ul>

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