This fails on the second line…
REXML::Document.new('<b>&</b>') REXML::Document.new('<b> </b>')
…with Undeclared entity ’ ’
If I make that real XHTML, with a DOCTYPE and a html xmlns="…xhtml",
I fixed the itself by adding  to the DOCTYPE.
This seems extremely tacky, because it disregards all the other fun
(I suspect REXML with 1.8 did not have this issue.)
If I should call Entity.new(‘nbsp’, ’ '), I don’t understand how to .add
that before the call to .new, and I don’t understand how to easily add
HTML content to a preexisting Document object.