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In Base.replace, the method checks that self.parent is not None - 
however, it (unfortunately), breaks this very property itself if self is 
new, or self in new.
In particular, some fixers return node from transform if they don't want 
to do anything. In that case, self.parent gets set to None, even though 
new (which is self) is still connected to the tree.
This patch
a) short-cuts the case that self is new
b) arranges to set parent of the new nodes only after clearing parent of 
self