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I can trigger this exception by calling the normalize()
method on a childless DOM node such as Text.  A little
script is attached. 

The question is: what should normalize() do for a class such
as Text?  Looking at the DOM Level 1 specification,
.normalize() is actually part of the Element interface, not
Node, so things like Text.normalize() wouldn't work at all.

One solution: move normalize() onto the Element class. 
Possibly backwards-incompatible.

Another solution: add a normalize() method to the Childless
mix-in that raises an exception.